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ANATOMY  AND  HISTOLOGY 


MOUTH  AND  TEETH 


I.  NORMAN  BROOMELL,  D.D.S. 

PROFESSOR     OF     DENTAL     ANATOMY,    DENTAL     HISTOLOGY,  AND     PROSTHETIC     TECHNICS     IN     THE 
PENNSYLVANIA     COLLEGE     OF     DENTAL     SURGERY,    PHILADELPHIA 


SeconJ)  Edition,  IRevlseJ)  anb  ]Enlarge&,  witb 
337  miustrations 


PHILADELPHIA 

P.    BLAKISTON'S    SON    &    CO. 

IOI2     WALNUT    STREET 
1902 


Copyright,  igoz,  by  P.  Blakiston's  Son  &  Co. 


Press  of  Wm.  F.  Fell  &  Co, 

I220-24  SANSOM   ST., 


TO 

C.   N.   PEIRCE,   D.D.S. 

AS    A    SOUVENIR    OF    A     LONG    AND    VALUED     FRIENDSHIP     AND    A 

TESTIMONY  OF   ESTEEM    FOR    HIS    PROFESSIONAL 

AND     PRIVATE     WORTH 

?Ibis  IDolume  is  IRcapcctfuUg  DeOicateO 

BY   THE   AUTHOR 


PREFACE  TO  SECOND    EDITION. 


In  submitting  to  the  dental  profession  a  revised  edition  of 
this  work,  the  author  has  embraced  the  opportunity  to  make 
such  corrections  and  additions  as  were  deemed  advisable  to  fur- 
ther increase  the  value  of  the  book,  and  to  make  it  worthy  of 
at  least  a  portion  of  the  approval  accorded  the  first  edition. 
The  general  plan  of  the  work  has  not  been  altered,  and  while 
some  exceptions  have  been  taken  to  including  within  its  pages 
chapters  devoted  to  the  general  anatomy  of  the  mouth  and  its 
associated  parts,  it  was  decided,  after  due  consideration,  to  allow 
this  to  remain. 

In  the  description  of  the  teeth,  the  terms  "superior"  and 
"inferior"  have  been  changed  to  "upper"  and  "lower,"  and 
the  term  "  palatal  "  as  applied  to  one  of  the  tooth  surfaces  has 
been  discarded,  and  the  word  "  lingual  "  substituted. 

Considerable  care  has  been  bestowed  upon  part  second,  and 
as  a  result  much  of  value,  both  as  to  descriptive  matter  and 
illustrations,  not  included  in  the  first  edition  will  be  here  found 
recorded.  A  chapter  dealing  with  the  Embryology  of  the  Mouth 
has  been  added,  as  has  also  a  short  chapter  on  Anomalies  of 
Tooth  Form  and  Structure. 

Fifty-three  new  illustrations  have  been  added,  all  of  which, 
with  one  or  two  exceptions,  are  the  original  work  of  the  author. 

Acknowledgment  is  due  the  "  Dental  Cosmos  "  and  "Items 
of  Interest  "  for  the  loan  of  cuts  in  their  possession,  and  to 
Miss  Emma  A.  Graham,  who  so  faithfully  assisted  in  the  work 
of  revision. 

The  author  indulges  the  hope  that  his  efforts  to  improve  the 
book  will  be  found  satisfactory  to  its  readers. 

1420  Chestnut  Street,  Philadelphia, 
Angus/,  igoz. 


PREFACE  TO  THE  FIRST  EDITION. 


In  the  preparation  of  this  work  it  has  been  the  aim  of  the 
author  to  systematically  describe  those  parts  of  human  anatomy 
which  come  directly  under  the  care  of  the  stomatologist.  In  the 
earlier  chapters,  which  are  devoted  to  a  gross  description  of  the 
mouth  and  those  tissues  which  enter  into  its  construction,  there 
has  been  no  attempt  at  originality  other  than  in  the  arrangement, 
which  includes  a  complete  description  of  one  part  before  another 
is  taken  up. 

In  the  writing  and  classification  of  the  succeeding  chapters 
the  writer  has  attempted  what  others,  though  wiser  and  better 
qualified,  appeared  unwilling  to  undertake,  and  it  is  from  the 
works  of  such  as  these  that  the  foundation  for  the  present 
work  has  been  derived. 

Within  the  last  few  years  the  progress  in  nearly  every  branch 
of  dental  education  has  made  a  work  of  this  character  an  imper- 
ative want.  Dental  therapeutics  and  dental  chemistry  have 
been  well-nigh  reconstructed,  while  the  investigations  of  the 
microscopist  and  physiologist  have  brought  forth  many  valuable 
revelations.  Next  in  importance  has  been  the  advance  in,  or 
rather  the  introduction  of,  technic  teaching.  Considerable  space 
has,  therefore,  been  devoted  to  the  surface  anatomy  of  the 
individual  teeth,  with  a  hope  that  it  may  be  of  value  in  dental 
anatomy  technic. 

While  in  one  or  two  instances  the  writer  has  departed  from 
the  field  assigned  as  a  text,  the  parts  thus  included  are  so 
closely  associated  with  the  mouth,  both  in  a  constructive  and  in 
a  functional  manner,  that  the  work  would  be  lacking  in  com- 
pleteness if  they  were  omitted. 

The  illustrations  are,  with  but  few  exceptions,  the  original 
work  of  the  author,  being  reproduced  by  photograph  from  the 
actual  subject.     In  many  instances  dissections  were  required  to 


reveal  the  parts,  this  being  particularly  true  of  those  illustrations 
included  in  the  chapter  on  the  Development  of  the  Teeth,  about 
one  hundred  dissections  being  required  to  accomplish  the  pur- 
pose. In  preparing  the  illustrations  descriptive  of  the  various 
surfaces  of  the  individual  teeth,  the  progress  of  the  work  was 
materially  interfered  with  by  the  difficulty  experienced  in  secur- 
ing normal  teeth  out  of  the  mouth  ;  may  their  number  ever 
grow  less. 

The  author  desires  to  thus  publicly  acknowledge  obligations 
to  the  works  of  Tomes,  Black,  Morris,  Stohr,  Klein,  and  Strieker. 
He  is  also  indebted  to  Prof  A.  P.  Brubaker  and  to  Dr.  C.  P. 
Shoemaker  for  valuable  assistance  rendered,  and  to  P.  Blakiston's 
Son  &  Co.  for  their  many  courtesies  during  the  preparation  of 
the  volume. 

That  there  is  a  place  for  such  a  work  as  this  purports  to  be 
the  writer  has  but  little  doubt ;  that  the  following  pages  will 
fill  that  demand  is  his  earnest  dfesire,  and  it  remains  for  the 
reader  to  ascertain  how  far  these  demands  have  been  met  in 
the  direction  of  its  aim  and  endeavor. 

302  North  Fortieth  Street,  Oct.  20,  iSgS. 


TABLE   OF   CONTENTS. 


PART  I.— ANATOMY. 

CHAPTER  I. 

General  Description  of  the  Mouth.- — The  Buccal  Orifice  ;  The  Lateral 
Walls  of  the  Mouth  ;  The  Hard  Palate,  or  Dome  of  the  Mouth  ; 
The  Soft  Palate  and  Fauces  ;  The  Floor  of  the  Mouth  ;  The 
Tongue  and  its  Attached  Muscles, 17-50 

CHAPTER  II. 

The  Bones  of  the    Mouth. — The  Superior  Maxilla;    The    Palate 

Bones;  The  Inferior  Maxilla,  or  Mandible,       51-81 

CHAPTER  III. 

The  Temporomandibular  Articulation. — The  Muscles  of  Mastication,         S2-93 

CHAPTER  IV. 

General  Description  of  the  Teeth ;  The  Permanent  Teeth  ;  Classifi- 
cation, Surfaces,  etc.;  The  Roots  of  the  Teeth;  The  Dental 
Arch, 94-107 

CHAPTER  V. 
Occlusion  of  the  Teeth, 108-114 

CHAPTER  VI. 
The  Blood- and  Nerve-supply  to  the  Teeth,    .    .    ; 1 15-123 

CHAPTER   VII. 

Other  Structures  Within  the  Mouth. — The  Gums  ;  The  Mucous  Mem- 
brane;   The  Alveolodental  Membrane;   Glands,   Ducts,  etc.,  .     124-135 

CHAPTER  VIII. 

A  Description  of  the  Upper  Teeth  in  Detail. — Calcification, 
Eruption,  and  Average  Measurements ;  Their  Surfaces,  Ridges, 
Fossae,  Grooves,  etc., 136-207 

CHAPTER  IX. 

A  Description  of  the  Lower  Teeth  in  Detail. — Calcification,  Erup- 
tion, and  Average  Measurements ;  Their  Surfaces,  Ridges, 
Fossae,  Grooves,  etc. , 208-237 


X  TABLE   OF   CONTENTS. 

CHAPTER  X. 
The  Pulp-cavities  of  the  Teeth, 238-267 

CHAPTER  XI. 

The  Deciduous  Teeth,  Their  Arrangement,  Occlusion,  etc.;  Calcifi- 
cation, Eruption,  Decalcification,  Shedding  Process,  and  Average 
Measurements  ;  Their  Surfaces,  Grooves,  Fossae,  Ridges,  and 
Pulp-cavities, 268-285 

CHAPTER  Xn. 

Development  of  the  Teeth. — The  Dental  Germs,  Enamel  Organ, 
and  Dentin  Organ  ;  The  Dental  Follicle;  Calcification,  Erup- 
tion, etc., 286-343 


PART  ll.-HISTOLOGY. 

CHAPTER  I. 

General  Histology. — The  Tissues  of  the  Body ;  The  Epithelial 
Tissues ;  Connective  Tissues ;  Muscular  Tissues,  and  Nervous 
Tissues, 344-363 

CHAPTER  II. 

The  Mucous  Membrane  of  the  Mouth. — Of  the  Lips;  Of  the  Cheeks  ; 
Of  the  Gums;  Of  the  Roof  of  the  Mouth,  Hard  and  Soft 
Palate;  Of  the  Floor  of  the  Mouth  and  the  Tongue, 364-375 

CHAPTER  III. 

Glands  and  Ducts  of  the  Mouth. — Of  the  Lips  ;   Of  the  Cheeks  ;  Of 

the  Hard  and  Soft  Palate  ;  Of  the  Tongue  ;  The  Salivary  Glands,     376-384 

CHAPTER  IV. 

Muscular  Tissues  of  the  Mouth.— Of  the  Lips  ;  Of  the  Cheeks ;  Of 

the  Soft  Palate  ;  Of  the  Tongue, 385-388 

CHAPTER  V. 

Tissues  of  the  Teeth. — Enamel ;  Dentin  ;  Cementum  ;  The  Tooth- 
Pulp  ;  The  Alveolodental  Membrane, 389-454 

CHAPTER  VI. 
Embryology  of  the  Mouth  and  Teeth, 455-480 

CHAPTER  VII. 
Anomalies  of  the  Teeth, .     481-491 


Index, 493 


ANATOMY    AND    HISTOLOGY 


MOUTH  AND  TEETH 


PART   I.— ANATOMY. 


CHAPTER  I. 

GENERAL  DESCRIPTION  OF  THE  MOUTH.— THE  BUCCAL  ORIFICE 
(THE  LIPS).— THE  LATERAL  WALLS  OF  THE  MOUTH  (THE  CHEEKS). 
-THE  HARD  PALATE  OR  DOME  OF  THE  MOUTH.-THE  SOFT 
PALATE  AND  FAUCES.— THE  FLOOR  OF  THE  MOUTH.-THE 
TONGUE   AND   ITS  ATTACHED   MUSCLES. 

The  mouth  (Fig.  i)  {stoma,  pL  stoinata)  is  the  entrance  or  gate- 
way to  the  ahmentary  canal,  and  is  situated  between  the  supe- 
rior and  inferior  ma.Killary  bones  and  their  attached  tissues.  It 
contains  the  active  organs  of  mastication,  the  teei/i,  the  organs 
of  taste,  of  which  tlie  tongue  is  chief  together  with  some  of  the 
parts  which  assist  in  articulate  speech.  Anatomists  usually 
divide  this  cavity  into  two  compartments,  the  teeth  serving  to 
separate  one  from  the  other,  the  inner  space  being  called  the 
month,  while  that  between  the  teeth  and  lips  or  cheeks  is  known 
as  the  vestibule  of  the  mouth.  In  this  description  all  that  space 
bounded  anteriorly  by  the  lips,  posteriorly  by  the  pillars  of  the 
fauces,  and  latterly  by  the  cheeks,  will  be  considered  as  a  single 
cavity,  and  the  organs  and  structures  contained  therein,  together 
with  all  parts  directly  interested  in  its  formation,  will  constitute 
a  text  for  this  work.  The  entrance  to  the  cavity  of  the  mouth  is 
formed  by  a  freely  movable  transverse  orifice  or  slit,  the  buccal 
2  17 


orifice,  while  it  communicates  with  the  pharynx  posteriorly 
through  the  fauces.  Entering  into  the  construction  of  the  mouth 
and  assisting  in  the  performance  of  its  functions  are  bones, 
ligaments,  muscles,  blood-vessels,  nerves,  glands,  ducts,  etc., 
each  of  which  will  be  described  in  turn. 


THE    BUCCAL    ORIFICE, 

or  entrance  to  the  cavity  of  the  mouth,  is  a  transverse  opening- 
somewhat  variable  in  extent,  the  extremities  of  which  are  known 
as  the  corners  or  angles  of  the  mouth.     The  orifice  is  bounded 


Fig,  I. — A  General  Vjew  of  the  Mouth. 


by  two  fleshy  folds,  the  upper  and  lozi<er  lips  [labia),  the  former 
usually  being  in  the  form  of  a  double  curve,  coming  together  at  the 
median  line  and  forming  a  small  teat  or  tubercle,  while  the  latter 
is  made  up  of  a  single  curve  extending  from  angle  to  angle. 
While  this  general  descripdon  applies  to  the  labial  forms  most 
frequently  met  with,  it  must  not  be  mistaken  for  a  constant  con- 


THE   BUCCAL   ORIFICE.  19 

dition.  In  some  instances  the  lips  are  tliin,  with  straight  parallel 
margins,  firmly  set  against  the  teeth,  and  seldom  separated  from 
each  other  when  at  rest.  In  another  class  they  are  thick,  full, 
and  prominent,  with  their  margins  strongly  curved,  resting 
lightly  against  the  teeth,  and  more  or  less  separated  from  each 
other  during  rest.  Accompanying  the  extreme  as  well  as  the 
intervening  conditions  are  various  other  peculiarities,  such  as 
the  color,  the  rigidity  or  flexibility  of  the  muscular  structure, 
etc.  The  upper  lip  generally  overhangs  the  lower,  but  in  some 
instances  the  lower  lip  is  the  most  prominent.  Externally  the 
lips  are  covered  by  the  common  integument,  internally  and  over 
their  contiguous  surfaces  by  a  continuation  of  the  integument, 
the  mucous  membrane.  Between  the  external  and  internal 
coverings  and  forming  the  substance  of  these  fleshy  folds  are 
muscular  fibers  in  which  are  imbedded  numerous  blood-vessels, 
nerves,  and  glands  (Jabial  glands).  By  the  various  muscles 
which  enter  into  their  construction  the  lips  are  attached  to  the 
surfaces  of  the  maxillary  bones. 

The  integument,  or  external  covering  of  the  lips,  is  similar 
to  the  skin  covering  other  parts  of  the  body.  In  the  male  it  is 
subject  to  a  peculiar  change  and  modification  of  its  outer  layer, 
resulting  in  the  production  of  a  hairy  growth. 

The  mucous  membrane,  or  internal  covering  of  the  lips, 
the  beginning  of  which  is  strongly  manifest  by  its  bright-red 
color,  is  without  moisture  on  the  contiguous  surfaces,  is  ex- 
tremely sensitive,  and  contains  a  number  of  vascular  papillae, 
many  of  which  are  accompanied  by  nerve  terminals.  Mucous 
membranes  are  described  as  lining  certain  cavities  or  tracts,  as 
the  digestive  tract,  the  respiratory  tract,  and  the  genito-urinary 
tract,  and  it  is  upon  the  contiguous  surfaces  of  the  lips  that  the 
digestive  tract  begins.  The  line  of  junction  between  the  integu- 
ment and  the  mucous  membrane,  is  quite  variable  in  form,  but 
usually  corresponds  with  the  general  curvature  of  the  lips. 
Internally  at  the  median  line  each  lip  is  provided  with  a  pro- 
nounced fold  of  mucous  membrane,  which  is  attached  to  the 
basal  portion  of  the  gum,  the  fremim  of  the  lip  (fraenum  labium 
superloris  and  inferioris),  which,  in  a  measure,  check  the  move- 
ments of  the  lips. 


20  ANATOMY. 

Muscles  of  the  Lips. 

The  muscular  fibers  within  the  substance  of  the  lips  are  prin- 
cipally those  of  a  single  muscle,  the  orbicularis  oi'is,  but  asso- 
ciated with  it  is  a  portion  of  the  elevator  and  depressor  muscles 
of  the  lips,  the  levator  labii  superioris  aliEqiie  nasi,  levator  labii 
siiperioris,  depressor  labii  inferioris  or  qiiadratus  luenti,  and  the 
zygoinaticus  minor. 

Orbicularis  Oris. — This  is  the  sphincter  muscle  which  sur- 
rounds and  controls  the  buccal  aperture.  In  form  it  is  an  oval 
sheet  with  the  long  axis  placed  transversely,  the  fibers  being 
continued  from  one  lip  to  the  other  by  passing  around  the 
angles  of  the  mouth.  It  is  divided  into  an  internal  or  labial 
portion,  and  an  external  or  facial  portion.  The  labial  portion 
forms  the  red  part  of  the  lips,  and  has  no  bony  attachment 
except  through  the  medium  of  the  adjacent  muscles.  The 
external  or  facial  portion  forms  the  deeper  layer  and  blends 
with  the  surrounding  muscles,  works  in  conjunction  with  them, 
and  is  provided  with  the  following  small  bony  attachments  : 
The  nasolabial  slips  are  attached  to  the  septum  of  the  nose, 
other  fibers  are  attached  to  the  incisive  fossa  of  the  superior 
maxilla  over  the  position  of  the  lateral  incisor  tooth,  and  to  the 
incisive  fossa  of  the  inferior  maxilla  near  the  socket  of  the  lateral 
incisor  or  cuspid  tooth. 

Structure. — The  muscle  consists  of  three  sets  of  fibers,  one  of 
which  runs  transversely,  one  in  a  vertical,  and  one  in  an  antero- 
posterior direction.  The  transverse  set  is  continuous  with  the 
fibers  of  the  buccinator  or  cheek  muscle,  and  forms  the  greater 
part  of  the  muscle.  The  red  or  labial  portion  of  the  muscle  is 
also  formed  from  the  same  fibers,  while  the  vertical  fibers  form 
the  superficial  part  of  the  facial  portion  and  are  continuous  with 
the  fibers  of  the  levator  and  depressor  muscles.  Some  of  these 
latter  fibers  pass  around  the  corners  ot  the  mouth,  thus  becom- 
ing transverse,  those  from  above  passing  to  the  lower  lip,  while 
those  from  below  pass  to  the  upper  lip.  The  anteroposterior 
fibers  pass  from  before  backward  between  the  transverse  fibers, 
and  unite  the  mucous  membrane  to  the  skin.  These  are  chiefly 
found  in  the  labial  portion  of  the  muscle. 

Relations. — The   inner   margin   of   the   superficial   surface   is 


THE   BUCCAL  ORIFICE.  21 

closely  connected  with  the  integument,  while  superimposed 
between  this  and  the  outer  portion  is  a  layer  of  fatty  tissue. 
Upon  the  deep  surface  lies  the  mucous  membrane  of  the  mouth, 
separated  from  the  muscular  tissue  by  blood-vessels,  mucous 
glands,  and  small  salivary  glands. 

Action. — To  bring  the  lips  together,  to  draw  the  upper  lip 
downward,  and  the  lower  lip  upward  ;  to  draw  together  the 
corners  ot  the  mouth  ;  to  throw  both  lips  outward  ;  to  draw 
them  back  against  the  teeth,  and  to  oppose  the  action  of  all 
other  muscles  that  blend  into  it  and  inclining  to  draw  it  in 
various  directions. 

Levator  Labii  Superioris  Alaeque  Nasi. — As  its  name 
implies,  this  muscle  is  an  elevator  of  the  upper  lip  and  the  wing 
of  the  nose.  It  is  one  of  the  superficial  facial  muscles,  is  thin 
and  triangular,  and  is  situated  by  the  side  of  the  nose,  extending 
from  the  infra-orbital  ridge  to  the  upper  lip. 

Origin. — From  the  nasal  process  of  the  superior  maxilla  near 
its  orbital  margin. 

Insertion.— Vxoxw  its  origin  it  passes  almost  directly  down- 
ward, dividing  into  two  portions,  the  smaller  of  which  is  inserted 
into  the  nasal  wing,  while  the  larger  portion  is  prolonged  down- 
ward, blending  into  the  orbicularis  oris  and  levator  labii  superi- 
oris, and  forming  a  part  of  the  substance  of  the  upper  lip. 

Relations. — Superficially,  by  the  integument ;  deeply,  by  the 
levator  anguli  oris  and  compressor  narium. 

Action. — By  its  smaller  and  shorter  portion  to  raise  the  wing 
of  the  nose  and  to  dilate  the  nostril  ;  by  its  larger  and  longer 
portion  to  elevate  the  inner  half  of  the  upper  lip. 

Levator  Labii  Superioris. — This  muscle  belongs  to  the 
superficial  layer,  and  derives  its  name  from  its  action. 

Origin. — From  the  facial  surface  of  the  superior  maxilla,  at  a 
point  between  the  orbital  cavity  and  the  infra-orbital  foramen. 
Also  by  the  attachment  of  a  few  fibers  to  the  malar  bone. 

Insertion. — Passing  downward  and  inward,  it  is  inserted  into 
the  orbicularis  oris  and  the  integument  of  the  upper  lip.  Near 
its  lower  third  it  joins  the  levator  labii  superioris  alaeque  nasi, 
and  acts  in  conjunction  with  it.      Occasional!)-  it  is  reinforced  by 


fibers  from  the  orbicularis  palpebrarum,  which  it  receives  at  its 
outer  border. 

Relations. — Superficially,  by  the  orbicularis  palpebrarum  and 
the  integument ;  deeply,  by  the  levator  anguli  oris  ;  the  com- 
pressor nasi  at  its  origin,  and  by  the  infra-orbital  vessels  and 
nerves. 

Action. — To  elevate  the  upper  lip. 

Levator  Labii  Inferioris. — This  muscle,  also  known  as 
levator  menti,  lies  immediately  beneath  the  mucous  membrane 
of  the  lower  lip,  and  can  best  be  dissected  by  everting  the  lip 
and  lifting  off  the  membrane. 

Origin. — From  the  incisor  fossa  of  the  lower  jaw  at  its  upper 
border. 

Insertion. — Into  the  integument  of  the  skin. 

Relations. — With  the  labial  integument,  the  lower  border  of 
the  orbicularis  oris  muscle,  and  its  superficial  surface  with  the 
oral  mucous  membrane.  Deeply  it  is  in  close  contact  with  the 
periosteum  and  the  depressor  labii  inferioris. 

Action. — To  raise  and  cause  to  protrude  the  integument  ot 
the  chin. 

Depressor  Labii  Superioris. — This  small  muscle  with  its 
fellow  of  the  opposite  side  is  sometimes  found  within  the  mucous 
membrane  forming  the  frenum  of  the  upper  lip. 

Origin. — It  arises  from  the  incisive  fossa  along  its  lower  mar- 
gin and  some  of  its  fibers  are  attached  to  that  part  of  the  alve- 
olar process  closely  associated  with  the  fossa. 

Insertion. — From  the  point  of  origin  the  fibers  pass  upward 
and  are  attached  to  the  lower  border  of  the  nostrils  and  parti- 
tion of  the  nose.  Some  of  the  fibers  of  this  muscle  are  also 
attached  to  the  integument  covering  the  wing  of  the  nose.  The 
balance  pass  downward  and  mingle  with  the  fibers  of  the  mus- 
cular structure  of  the  upper  lip. 

Relations. — At  its  point  of  origin  the  fibers  are  closely  asso- 
ciated with  the  mucous  membrane  forming  the  gums.  Above 
this  the  muscular  structure  of  the  lip  overlies  these  fibers.  Deeply 
it  rests  upon  the  surface  of  the  superior  maxilla,  and  joins  its 
fellow  of  the  opposite  side  at  the  median  line. 

Action. — To  depress  the  upper  lip. 


THE    BUCCAL   ORIFICE.  23 

Depressor  Labii  Inferioris,  or  Quadratus  Menti. — The 

name  of  this  muscle  is  derived  from  its  form  and  action.  It 
belongs  to  the  second  layer  of  facial  muscles,  Is  quadrilateral  in 
shape,  and  consists  of  parallel  fibers  which  meet  above  in  the 
median  line. 

Origin. — At  the  outer  aspect  of  the  lower  border  of  the  infe- 
rior maxilla,  from  a  point  near  the  symphysis  to  the  space  be- 
neath the  first  bicuspid  tooth. 

Insertion. — Its  fibers  pass  upward  and  inward,  and  after  unit- 
ing with  its  fellow  of  the  opposite  side,  blend  into  the  body  of 
the  orbicularis  oris  of  the  lower  lip. 

Relations. — By  its  superficial  surface  with  the  integument  and 
a  portion  of  the  depressor  anguli  oris  ;  deeply,  with  the  mental 
nerve  and  vessels,  a  portion  of  the  orbicularis  oris,  the  mucous 
membrane  lining  the  lower  lip,  and  the  labial  glands. 

Actioji. — To  draw  down  and  somewhat  evert  the  lower  lip. 

Zygomaticus  Minor. — An  extremely  slender  muscle  belong- 
ing to  the  superficial  set  of  facial  muscles.  It  is  closely  associ- 
ated with  a  larger  muscle,  the  zygomaticus  major,  belonging  to 
the  angular  series,  to  be  described  in  connection  with  the  mus- 
cles of  the  cheek. 

Origin. — From  the  anterior  inferior  part  of  one  of  the  facial 
bones, — the  malar, — close  to  its  junction  with  the  superior  maxilla. 

Insertion. — It  passes  downward  and  forward,  its  fibers  becom- 
ing lost  in  the  special  elevator  muscle  of  the  upper  lip  about 
midway  between  the  median  line  and  the  angle  of  the  mouth. 

Relations. — Superficially,  by  the  integument,  by  its  deep  sur- 
face with  the  levator  anguli  oris,  facial  portion  of  the  orbicularis 
oris,  and  the  infra-orbital  branch  of  the  facial  nerve. 

Action. — To  elevate  and  somewhat  evert  the  upper  part  of 
the  lip. 

The  Blood-supply  to  the  Lips. 

The  blood-supply  to  the  lips  is  principally  through  the  superior 
and  inferior  coronary  arteries,  both  of  which  are  branches  of 
the  facial  artery.  In  addition  to  these  the  inferior  labial  artery 
and  the  sub-mental,  also  branches  of  the  facial,  and  the  mental 
branch  of  the  inferior  dental  artery  supply  a  part  of  the  lower  lip. 


The  superior  coronary  artery  courses  along  the  inferior 
margin  of  the  upper  Hp,  between  the  mucous  membrane  and 
the  fibers  of  the  orbicularis  oris  muscle.  At  the  median  line  it 
anastomoses  with  its  fellow  of  the  opposite  side. 

The  inferior  coronary  artery,  somewhat  smaller  than  the 
superior,  supplies  the  lower  lip  by  coursing  through  its  substance 
in  a  manner  similar  to  the  superior  coronary  and  also  anasto- 
moses with  its  fellow  of  the  opposite  side  at  the  median  line. 

Course  of  the  Blood  From  the  Heart  to  the  Lips. — From 
the  heart  to  the  aorta,  to  the  common  carotid,  to  the  external 
carotid,  to  the  facial,  to  the  superior  and  inferior  coronary  and 
the  inferior  labial  arteries.  After  passing  through  the  labial 
capillaries  the  blood  is  returned  to  the  heart  through  the 
superior"  and  inferior  coronary  veins,  and  the  larger  veins  of 
which  they  are  branches. 

Nerves  of  the  Lips. 

The  general  nerve-supply  to  the  lips  is  principally  by  small 
branches  of  the  infra- orbital  nerve  for  the  upper  lip,  and  by 
branches  of  the  mental  nerve  for  the  lower  lip.  The  bttccal  and 
superior  maxillary  branches  of  the  lower  division  of  the  facial 
nerve  supply  the  orbicularis  oris  muscle  ;  the  upper  division  of 
the  facial  nerve  sends  branches  which  supply  the  levator  labii 
superioris  alajque  nasi,  as  well  as  the  levator  labii  superioris 
and  the  zygomaticus  minor,  while  the  superior  maxillary  branch 
of  the  lower  division  of  the  facial  supplies  the  depressor  labii 
inferioris. 

THE   LATERAL  WALLS  OF  THE   MOUTH. 

The  Cheeks  [bucccs). 

The  cheeks  are  continuous  with  and  similar  in  structure  to 
the  lips,  being  covered  internally  by  mucous  membrane  and  ex- 
ternally by  the  common  integument.  Immediately  beneath  the 
mucous  membrane  are  a  number  of  transverse  muscular  fibers, 
covered  externally  by  a  layer  of  subcutaneous  fat,  and  lying 
between  this  and  the  integument  other  muscular  tissue,  the 
fibers  of  which  radiate  in  various  directions,  according  to  the 
action  of  the  muscle  to  which  thev  belong.     Besides  muscular 


THE   LATERAL   WALLS   OF   THE    MOUTH.  25 

and  fatty  tissue,  there  are  imbedded  within  the  substance  of  the 
cheek  blood-vessels,  nerves,  and  glands.  The  fatty  tissue 
spoken  of  as  intervening  between  the  muscular  fibers  gives  to 
the  cheek  the  fullness  and  rotundity  desired  by  so  many  but 
possessed  by  so  few. 

The  integument,  or  external  covering  of  the  cheek,  is  similar 
in  structure  to  the  skin  covering  other  parts  of  the  body,  and, 
like  the  lips  in  the  male,  is  productive  of  a  hairy  growth. 

The  mucous  membrane,  or  internal  covering  of  the  cheek, 
is  similar  to  that  of  the  lips,  containing  numerous  glands  {Imccal 
glands)  which  are  almost  identical  to,  but  smaller  than,  the  labial 
glands.  In  addition  to  the  buccal  glands  which  are  distributed 
over  the  entire  membrane,  there  are  about  five  of  larger  size, 
which  open  into  the  mouth  in  the  region  of  the  molar  teeth,  and 
are  called  molar  glands   (see  Glands  of  the  Mouth). 

The  Muscles  of  the  Cheeks. 

The  transverse  muscular  fibers  referred  to  as  being  immedi- 
ately beneath  the  mucous  membrane  are  those  of  the  bticcmator, 
a  muscle  named  from  its  action,  that  of  being  the  chief  muscle 
employed  by  the  trumpeter.  External  to  the  buccinator  is  the 
masseter,  one  of  the  muscles  of  mastication,  the  elevator  and 
depressor  muscles  of  the  angle  of  the  mouth,  the  levator  angtdi 
oris,  and  the  depressor  angnli  oris,  and  the  dermal  muscles, 
zygoviaticiis  majo7^  and  zygoniatiais  minor,  and  the  risomts. 

Buccinator. — This  muscle  forms  the  greater  portion  of  the 
lateral  wall  of  the  mouth.  It  is  deep-seated  in  the  cheek,  being- 
one  of  the  third  stratum  of  facial  muscles. 

Origin. — The  fibers  are  distinct  in  their  origin  from  a  part  ot 
the  alveolar  process  of  the  superior  maxillary  bone,  at  a  point 
immediately  over  the  second  and  third  molar  teeth,  from  the 
anterior  border  of  the  pterygomaxillary  ligament,  a  narrow 
band  of  tendinous  fibers  or  raphe  extending  from  the  pterygoid 
plate  of  the  sphenoid  bone  to  the  mylohyoid  ridge  of  the  interior 
maxilla  near  the  position  of  the  third  molar  tooth.  Some  of  its 
fibers  also  arise  from  the  outer  wall  of  the  alveolar  process  of 
the  inferior  maxilla  below  the  second  and  third  molars. 

Insertion. — The  fibers    pass  forward   and    converge  as  the)" 


reach  the  lateral  margins  of  the  orbicularis  oris  ;  here  the  fibers 
of  the  upper  portion  pass  downward  and  blend  into  the  mus- 
cles of  the  lower  lip,  while  the  lower  fibers  pass  upward  and 
blend  into  those  of  the  upper  lip.  Those  fibers  which  arise  from 
the  inferior  maxilla  pass  forward  and  also  blend  into  the  lower 

lip- 

Relations. — Superficially,  by  the  skin  and  subcutaneous  iat, 
the  duct  of  Steno,  the  masseter  muscle,  a  portion  of  the  angular 
group,  and  the  facial  artery  and  vein.  Passing  over  it  are 
branches  of  the  facial  and  buccal  nerves,  also  a  layer  of  deep 
fascia  continuous  with  that  which  covers  the  upper  part  of  the 
pharynx.  By  its  deep  surface  it  is  in  relation  with  the  mucous 
membrane  and  buccal  glands. 

Action. — To  draw  outward  or  backward  the  angles  of  the 
mouth,  thus  enlarging  the  buccal  orifice  and  pressing  the  lips 
tightly  against  the  teeth ;  to  force  the  food  between  the  occlusal 
surfaces  of  the  molar  and  bicuspid  teeth  during  mastication  ;  to 
diminish  the  concavity  of  the  cheek,  compressing  the  air  con- 
tained therein  and  forcing  it  forward.  It  becomes  an  auxiliary 
in  deglutition  by  shortening  the  cavity  of  the  pharynx  from 
before  backward,  through  its  connection  with  the  superior  con- 
strictor. 

Masseter. — This  muscle  is  placed  immediately  external  to 
the  buccinator,  and  is  one  of  the  principal  muscles  of  mastica- 
tion. It  is  short,  thick,  and  somewhat  quadrate  in  form,  and  is 
composed  of  two  sets  of  fibers,  superficial  and  deep.  The  fibers 
of  the  former  are  directed  obliquely  downward  and  backward  ; 
those  of  the  latter,  which  are  much  shorter,  pass  almost  verti- 
cally downward. 

Origin. — The  superficial  layer,  from  the  malar  process  of  the 
superior  maxilla,  and  from  the  anterior  portion  of  the  zygomatic 
arch  of  the  malar  bone.  The  deep  layer  from  the  posterior 
third  of  the  zygomatic  arch,  as  well  as  from  the  greater  part  ot 
its  inner  surface. 

Insertion. — The  superficial  fibers  are  inserted  into  the  ramus 
and  angle  of  the  Inferior  maxilla,  and  the  deep  fibers  into  the 
upper  half  of  the  outer  surface  of  the  ramus. 

Relations. — By    its    external    surface    with    the    zygomaticus 


THE   LATERAL   WALLS   OF   THE   MOUTH.  27 

major,  risorius,  and  platysma  myoides  muscles,  the  parotid 
gland  and  its  duct ;  by  the  transverse  facial  artery,  the  facial 
vein,  and  facial  nerve,  and  by  the  integument.  By  its  internal 
surface  with  the  ramus  of  the  inferior  maxilla,  a  mass  of  fat 
which  separates  it  from  the  buccinator,  and  with  the  temporal 
muscle.  Its  posterior  margin  is  in  relation  with  the  parotid 
gland,  and  its  anterior  with  the  facial  artery  and  vein. 

Action. — The  principal  action  of  this  muscle  is  to  close  the 
jaw  and  to  draw  it  slightly  forward.  (For  further  description, 
see  Muscles  of  Mastication,  part  i,  chap,  iii.) 

The  Angular  Series. — The  remaining  muscles  of  the  cheek 
are  those  of  the  angular  series,  or  those  muscles  which  are 
inserted  into  the  angle  of  the  mouth,  two  coming  obliquely  from 
above, — the  levator  angiili  oris  and  the  zygomaticus  major, — one 
running  almost  horizontally  forward, — the  risorius, — and  one 
ascending  from  below, — the  depressor  anguli  oris. 

Levator  Anguli  Oris. — This  muscle,  which  receives  its  name 
from  its  action,  belongs  to  the  second  layer  of  facial  muscles. 
It  is  formed  in  the  shape  of  a  triangular  sheet. 

Origin. — From  the  canine  fossa  of  the  superior  maxilla,  im- 
mediately below  the  infra-orbital  foramen. 

Insertion. — Passing  downward  and  outward  it  is  inserted  into 
the  angle  of  the  mouth,  its  fibers  blending  with  those  of  the 
orbicularis  oris  and  the  other  angular  muscles. 

Relations. — Superficially,  with  the  levator  labii  superioris,  the 
zygomaticus  minor,  and  the  infra-orbital  vessels  and  nerves ; 
deeply,  with  the  facial  portion  of  the  orbicularis  oris  anci  bucci- 
nator muscles,  and  the  mucous  membrane  of  the  mouth. 

Action. — Especially  to  elevate  the  angle  of  the  mouth,  and  to 
assist  in  drawing  these  angles  inward,  decreasing  the  size  of  the 
buccal  orifice. 

Zygomaticus  Major. — This  muscle,  the  companion  of  which 
has  been  described  in  connection  with  the  muscles  of  the  lips, 
belongs  to  the  first  facial  layer.  It  is  composed  of  a  long,  fleshy 
band  of  muscular  fibers,  which  run  direct  from  their  point  of 
origin  to  their  point  of  insertion. 

Origin. — From  the  malar  bone,  in  close  proximity  to  the 
zygomatic  suture. 


28  ANATOMY. 

Insertion. — From  its  origin  it  passes  obliquely  downward  to 
the  angle  of  the  mouth,  and  blends  into  the  fibers  of  the  orbicu- 
laris oris  and  depressor  anguli  oris. 

Relations. — Superficially,  with  the  skin  and  subcutaneous  fat; 
deeply,  with  the  malar  bone,  the  masseter  and  buccinator  mus- 
cles, the  facial  and  transverse  facial  arteries,  the  facial  vein,  and 
branches  of  the  facial  nerve. 

Action. — To  draw  upward  and  outward  the  angles  of  the 
mouth,  as  in  smiling  or  laughing.  By  contracting,  it  throws  into 
prominence  the  cheek  tissues  in  front  of  the  malar  bone,  and 
forces  the  lower  eyelid  upward.  When  acting  simultaneously 
with  its  fellow  of  the  opposite  side,  the  buccal  aperture  is 
widened,  and  the  upper  lip  is  elevated,  exposing  the  superior 
teeth. 

Risorius. — One  of  the  superficial  set  of  facial  muscles,  re- 
ceiving its  name  from  its  supposed  action  in  laughter  [ridere,  to 
laugh).  It  is  flat  and  ribbon-shaped,  and  is  frequently  very 
small  and  poorly  developed. 

Origin. — From  the  deep  fascia  covering  the  masseter  muscle 
and  parotid  gland,  some  of  its  fibers  occasionally  arising  from 
the  mastoid  process  of  the  temporal  bone. 

Insertion. — Passing  transversely  forward  and  inward  to  the 
angle  of  the  mouth,  its  fibers  blend  with  those  of  the  orbicularis 
oris,  and  the  depressor  anguli  oris. 

Relations. — Superficially,  with  the  integument  and  subcuta- 
neous fat ;  deeply,  with  the  masseter  and  buccinator  muscles,  the 
facial  artery  and  vein,  and  branches  of  the  facial  nerve. 

Action. — To  draw  the  angles  of  the  mouth  directly  outward, 
thereby  increasing  the  width  of  the  buccal  orifice. 

Depressor  Anguli  Oris. — Also  one  of  the  superficial  layer 
of  facial  muscles,  deriving  its  name  in  accordance  with  its  action 
upon  the  angle  of  the  mouth.  It  is  a  triangular-shaped  muscle 
with  its  base  below,  becoming  narrow  as  it  ascends. 

Origin. — From  the  lower  border  of  the  inferior  maxilla,  and 
from  its  external  oblique  line  below  the  cuspid,  bicuspid,  and 
first  molar  teeth. 

Insertion. — Passing  upward  and  inward  it  is  inserted  into  the 
integument  at  the  angle  of  the  mouth,  its  fibers  blending  into 


THE    LATERAL   WALLS   OF   THE    MOUTH.  29 

those  of  the  muscles  previously  described  as  coming  together  at 
this  point. 

Relations. — Externally,  with  the  integument ;  deeply  or  inter- 
nally, with  the  depressor  labii  inferioris,  the  buccinator,  and  the 
inferior  coronary  artery. 

Action. — To  draw  down  the  angle  of  the  mouth  and  to  slightly 
extend  it. 

Blood-supply  to  the  Cheeks. 

The  blood-supply  to  the  cheeks  is  principally  through  the 
facial  artery  and  its  direct  branches,  the  superior  and  inferior 
coronary,  the  transverse  facial  and  branches  from  the  internal 
maxillary. 

The  Facial  Artery  and  Branches. — The  facial  artery,  also 
called  the  external  maxillary,  enters  the  cheek  after  passing 
over  the  body  of  the  inferior  maxilla  at  the  anterior  edge  of  the 
masseter  muscle.  It  courses  obliquely  forward  and  upward 
through  the  substance  of  the  cheek,  until  it  reaches  the  inner 
angle  or  canthus  of  the  eye,  where  it  joins  the  nasal  branch  of 
the  ophthalmic  artery,  and  is  called  the  angular  artery.  Near 
the  center  of  the  cheek  the  iiiferior  labial  artery  is  given  off, 
which  passes  forward  and  downward  to  the  lower  lip,  but  sup- 
plies a  portion  of  the  cheek  in  so  doing.  Midway  between  the 
center  of  the  cheek  and  the  angle  of  the  mouth  the  sjcperior  and 
inferior  coronary  arteries  are  given  off,  supplying  that  part  ot 
the  cheek  immediately  adjacent  to  the  angle  of  the  mouth,  alter 
which  they  pass  on  to  supply  the  upper  and  lower  lips.  The 
masseteric  branch  is  given  off  in  the  immediate  center  of  the 
cheek,  at  a  point  immediately  below  the  inferior  labial,  passes 
directly  upward  over  the  masseter  muscle,  and  anastomoses  with 
branches  of  the  internal  maxillary  and  transverse  facial.  There 
are  also  given  off  from  the  main  trunk  near  the  center  of  the 
cheek  the  buccal  branches,  which  pass  upward  over  the  bucci- 
nator muscle,  and  also  anastomose  with  branches  of  the  internal 
maxillary  and  transverse  facial  arteries. 

The  Transverse  Facial  Artery. — This  is  the  largest  branch 
of  the  temporal  arter)'.  It  is  at  first  deeply-seated  in  the  sub- 
stance of  the  parotid  gland,  after  leaving  which  it  courses  trans- 


versely  over  and  supplies  the  masseter  muscle,  sends  off  small 
branches  which  supply  the  integument  of  the  cheek,  and  anas- 
tomoses with  the  buccal,  infra-orbital,  and  the  facial  arteries. 
Besides  the  arteries  already  named,  the  deeper  portions  of  the 
cheek  receive  blood  from  two  branches  of  the  internal  maxillary 
artery,  the  masseteric  branch  and  the  buccal  branch.  The 
former  supplies  the  masseter  muscle  and  anastomoses  with  the 
masseteric  branch  of  the  facial,  while  the  latter  supplies  the 
buccinator  muscle  and  anastomoses  with  the  buccal  branches 
of  the  facial. 

Course  of  the  Blood  From  the  Heart  to  the  Cheeks. — 
From  the  heart  to  the  aorta,  to  the  common  carotid,  to  the 
external  carotid,  to  the  facial  and  its  direct  branches,  or  from  the 
external  carotid  to  the  temporal,  to  the  transverse  facial  and 
branches. 

From  the  cheeks  the  blood  is  returned  to  the  heart  jarinci- 
pally  through  the  facial  vein,  a  division  of  the  anterior  super- 
ficial vein.  It  enters  the  cheek  at  a  point  midway  between  the 
lower  eyelid  and  the  wing  of  the  nose,  passes  obliquely  down- 
ward, being  in  close  contact  with  the  anterior  edge  of  the  mas- 
seter muscle  over  the  body  of  the  lower  jaw,  joining  the  inter- 
nal jugular  vein  in  the  neck.  The  transverse  facial  vein  which 
follows  the  course  of  the  transverse  facial  artery,  and  the  supe- 
rior and  inferior  coronary  veins  also  collect  and  convey  a  por- 
tion of  the  blood  from  the  cheeks  to  the  larger  veins  and  thence 
to  the  heart. 

Nerves  of  the  Cheeks. 

The  nerve-supply  to  the  cheeks  is  principally  from  the 
seventh  o'c  facial  nerve  and  its  branches,  the  buccal  branch  sup- 
plying a  greater  part  of  their  substance.  There  are  also  a  few 
fibers  of  the  infra-orbital  branch  of  the  seventh  nerve  distributed 
to  the  labiobuccal  region.  The  btucal  branch  of  the  lower 
division  of  the  facial,  also  the  buccal  branch  of  the  inferior  max- 
illary division  of  the  fifth  nerve,  supplies  the  buccinator  muscle. 
The  infra-orbital  branch  of  the  upper  division  of  the  facial  nerve 
supplies  the  zygomaticus  major  and  the  levator  anguli  oris  ;  the 
buccal  branch    supplies    the    risorius,    and   the    snpramaxillary 


THE   INTERIOR   OF   THE   MOUTH.  31 

branch  of  the  lower  division   of  the  facial  nerve  supplies    the 
depressor  anguU  oris. 

THE    INTERIOR    OF    THE    MOUTH. 

For  convenience  of  description  the  mouth  may  be  divided 
into  two  parts — a  superior  portion  and  an  inferior  portion.  In  dis- 
section this  division  may  be  accomplished  by  an  incision  begin- 
ning at  the  angles  of  the  mouth  and  carried  backward  and 
slightly  upward  through  the  substance  of  the  cheeks  until  the 
temporomaxillary  articulation  is  reached.  After  disarticulating 
this  joint  another  incision  is  made,  beginning  at  the  joint  on 
either  side,  carried  downward  and  forward,  then  obliquely  across 
the  throat,  until  the  two  come  together  at  the  median  line.  This 
latter  incision  must  be  deep  enough  to  completely  sever  the  tis- 
sues of  the  throat. 

The  superior  portion  of  the  mouth  contains  the  Jiard  palate, 
or  roof  of  the  mouth,  the  soft  palate,  and  the  sixteen  tipper  teeth, 
firmly  set  in  the  bone  and  surrounded  by  a  dense  fibrous  tissue 
— the  gums.  The  inferior  portion  contains  the  tongue  and  its 
attached  muscles,  forming-  the  floor  of  the  mouth,  the  sixteen 
loiver  teeth  and  the  gums  surrounding  them. 

THE  SUPERIOR  PORTION  OF  THE  MOUTH   (Fig.  2). 

The  osseous  framework,  or  base  upon  which  this  half  of  the 
mouth  is  constructed,  is  composed  of  a  part  of  four  bones — 
the  two  superior  maxillary,  or  upper  jaw  bones,  and  the  two 
palate  bones  (see  Bones  of  the  Mouth,  p.  51). 

The  Hard  Palate,  or  Roof  of  the  Mouth  (Fig.  2). 

This  is  formed  by  the  union  of  the  palatal  processes  of  the 
superior  maxillary  bones  and  the  horizontal  plates  of  the  two 
palate  bones  at  the  median  line.  It  is  limited  in  front  and  later- 
ally by  the  margins  of  the  alveolar  process,  or  that  portion  of 
the  bone  which  gives  support  to  the  teeth,  and  ends  posteriorly 
in  an  irregular  border,  to  which  is  attached  a  muscular,  mem- 
brane-like curtain — the  soft  palate.  The  hard  palate  is  covered 
throughout  by  a  thick  and  firm  mucous  membrane,  seldom  so 


highly  colored  as  the  membrane  lining  the  lips  and  cheeks. 
The  mucous  membrane  is  closely  adherent  to  the  bone  through 
its  covering,  the  periosteum.  In  the  center  of  the  hard  palate 
is  a  ridge  or  fold  of  mucous  membrane,  which  follows  the 
median  line  from  before  backward ;    this  is  called  the  palatal 


Fig.  2. — The  Superior  Portion  or  Roof  of  the  Mouth. 


raphe.  Anteriorly,  the  raphe  ends  in  a  small  papilla,  Avhich 
marks  the  opening  of  a  canal  in  the  bone — the  anterior  palatal 
canal.  Posteriorly,  the  raphe  usually  diminishes,  but  occasion- 
ally is  well  marked  through  the  whole  extent  of  the  hard  palate. 
Near  the  center  of  the  hard  palate  it  frequently  separates  into 


THE    INTERIOR    OF   THE   MOUTH.  33 

two  or  more  smaller  ridges,  which  are  proportionately  dimin- 
ished in  size,  and  are  continued  backward  side  by  side.  On 
either  side  of  this  central  ridge,  anteriorly,  the  mucous  mem- 
brane presents  a  number  of  fantastically  arranged  folds,  the 
palatal  rjt.g(E  (wrinkles).  These  folds  are  usually  quite 
numerous  and  prominent,  but  are  occasionally  almost  absent. 
The  nature  of  these  wrinkles  is  strongly  indicative  of  the  char- 
acter or  temperament  of  the  individual ;  thus,  in  the  four  basal 
temperaments  they  may  be  divided  as  follows  :  in  the  bilious, 
heavy  and  strong,  composed  of  angles  rather  than  curves  ;  in 
the  nervous,  few  in  number,  close  together,  not  prominent,  and 
composed  of  long  curves;  in  the  sanguine,  quite  numerous, 
fairly  prominent,  well  rounded  and  graceful  in  outline;  in  the 
lymphatic,  few  in  number,  flat,  widely  separated,  and  but  little 
curved.  Accompanying  these  varying  conditions  in  the  rugse 
will  be  found  a  corresponding  variation  in  the  raphe.  The  an- 
terior and  lateral  margins  of  the  mucous  membrane  covering 
the  hard  palate  form  the  palatal  portion  of  the  gums,  known  as 
the  gingiva  (the  gum)  or  gingival  border. 

In  figure  3  the  hard  palate  is  shown  with  its  membranous 
covering  removed.  It  will  be  observed  that  the  bony  plates  are 
perforated  by  numerous  small  foramina,  through  which  the  body 
of  the  bone  receives  its  nourishment,  broken  by  depressions  for 
the  accommodation  of  the  various  mucous  glands,  and  traversed 
by  longitudinal  grooves  which  give  lodgment  to  blood-vessels 
and  nerves. 

The  arch  formed  by  the  hard  palate  from  side  to  side  varies 
greatly  in  form,  imparting  much  knowledge  in  regard  to  the 
temperament  of  the  individual,  and  in  a  measure  controlling  the 
quality  of  the  voice.  Thus,  in  the  sanguine  temperament  the 
roof  of  the  mouth  presents  almost  a  perfect  oval.  In  the  bilious 
type  it  is  comparatively  high  and  flat,  extending  from  the  base 
of  one  alveolar  process  to  another,  from  which  point  it  descends 
abruptly  to  the  necks  of  the  teeth.  In  the  nervous  type  the 
roof  is  high  and  semi-elliptical  or  parabolic  in  shape,  and  in  the 
lymphatic  it  is  low  and  segmental  in  form. 

In  the  same  illustration  the  union  or  suture  between  the  two 
bones  may  be  observed  at  the  median  line.     Near  the  anterior 


third  of  this  central  suture  is  the  opening  of  the  incisive  or 
anterior  palatal  canal,  the  anterior  palatal  foramen,  the  location 
of  which  has  been  referred  to  in  the  description  of  the  mucous 
membrane.  Near  the  posterior  border,  and  situated  within  the 
suture  which  unites  the  superior  maxillary  bones  with  the  palate 
bones  (the  palatomaxillary  suture),  are  two  other  foramina,  the 
posterior  palatal ;  and  immediately  behind  these,  and  separated  by 
a  thin  ridge  of  bone,  are  the  accessory  palatal  foramina,  these 
being  in  the  tuberosity  of  the  palate  bones.     (For  further  descrip- 


FiG.  3. — The  Hard  Palate,  or  Roof  of  the  Mouth,  with  its  Membranous 
Covering  Removed. 

tion  oi  these  foramina,  see  Bones  of  the  Mouth,  p.  51.)  By  the 
vessels  and  nerves  which  enter  the  hard  palate  through  these 
various  foramina,  the  mucous  membrane  and  glands  receive  their 
blood-  and  nerve-supply. 

Blood-supply  to  the  Hard  Palate. — This  is  principally 
derived  from  the  posterior  or  descending  palatal  branch  of  the 
internal  maxillary  or  deep  facial  artery,  which  passes  downward 
in  the  posterior  palatal  canal  and  emerges  through  the  posterior 
palatal  foramen.  Immediately  on  reaching  the  palate  it  divides 
into  an  anterior  and  a  posterior  branch,  the  former  passing  for- 


THE   INTERIOR   OF  THE   MOUTH.  35 

ward  in  a  groove  provided  for  it  to  tlie  anterior  palatal  foramen, 
where  it  anastomoses  with  the  nasopalatal  artery.  The  groove 
in  which  the  artery  lies  in  its  passage  forward  is  usually  at  the 
base  of  the  alveolar  process,  and  in  some  instances  is  converted 
into  a  canal  for  a  part  of  its  length.  The  posterior  branches 
pass  backward  and  downward  to  supply  the  soft  palate.  In 
connection  with  supplying  the  hard  palate  proper,  this  artery 
carries  blood  to  the  palatal  alveolar  walls,  to  the  mucous  glands, 
the  mucous  membrane,  and  the  gums. 

Course  of  the  Blood  from  the  Heart  to  the  Hard 
Palate. — From  the  heart  to  the  aorta,  to  the  common  carotid,  to 
the  external  carotid,  to  the  internal  maxillary,  to  the  posterior  or 
descending  palatal  branch  of  the  latter.  From  the  hard  palate 
the  blood  is  returned  to  the  heart  by  the  superior  palatal  and 
inferior  or  descending  palatal  veins,  the  former  following  the 
course  of  the  superior  palatal  artery,  while  the  latter  originates 
at  a  point  near  the  junction  of  the  hard  and  soft  palates,  passes 
downward,  and  joins  the  facial  vein  below  the  body  of  the  inferior 
maxilla. 

Nerves  of  the  Hard  Palate. — The  nerves  of  the  hard 
palate  are  the  anterior  or  large  palatal  and  branches  from 
the  nasopalatal,  both  of  which  are  branches  of  the  spheno- 
palatal  (Meckel's)  ganglion.  The  anterior  palatal  nerve 
arises  from  the  inferior  angle  of  the  ganglion,  passes  down- 
ward, accompanied  by  the  descending  palatal  artery,  through 
the  posterior  palatal  canal,  from  which  it  emerges  at  the  pos- 
terior palatal  foramen.  From  this  point  it  passes  forward  in  a 
groove  of  the  hard  palate,  and  joins  the  nasopalatal  nerve  as 
it  emerges  from  the  anterior  palatal  foramen.  Accompanying 
this  nerve  in  its  course  through  the  posterior  palatal  canal  are 
other  branches  of  the  sphenopalatal  ganglion,  which  pass  to 
the  soft  palate,  and  will  be  described  in  that  connection.  The 
nerves  of  the  hard  palate  are  all  sensory,  and  filaments  are  dis- 
tributed to  the  mucous  membrane  and  glands  and  to  the  palatal 
portion  of  the  gums. 


THE  SOFT  PALATE  (Fig.  2). 

The  soft  palate  is  attached  to  the  posterior  border  of  the  hard 
palate,  from  which  it  is  continued  as  a  backward  prolongation. 
Hanging  downward,  with  its  free  borders  inclining  backward,  it 
may  be  considered  as  forming  a  part  of  the  posterior  boundary 
of  the  mouth.  It  partially  separates  the  mouth  from  the  nasal 
cavity  and  from  the  pharynx.  It  is  attached  laterally  to  the  walls 
of  the  pharynx,  while  its  lower  border  is  free.  The  substance  of 
the  soft  palate  is  composed  of  a  number  of  thin,  but  dense,  mus- 
cular fibers,  blood-vessels,  nerves,  and  mucous  glands,  the  latter 
being  similar  to  those  of  the  hard  palate.  The  anterior  surface 
of  this  muscular  curtain  is  concave,  directed  forward  and  down- 
ward, and  is  traversed  by  a  median  raphe.  The  posterior  sur- 
face is  conve.x,  directed  backward  and  upward,  and  is  continuous 
with  the  nasal  cavity.  Suspended  from  the  center  of  its  free 
border  is  a  small  rounded  or  conic  membranous  appendix,  the 
uvula,  and  passing  outward  from  the  base  of  this  at  each  side 
are  two  curved  folds  of  mucous  membrane,  which  extend  out- 
ward and  downward,  and  are  known  as  the  pillars  of  the  fauces. 
From  the  position  which  these  folds  occupy  they  are  divided  into 
the  antenor  and  the  posterior  pillars  of  the  fauces.  The  anterior 
pillar  is  formed  from  muscular  fibers  which  extend  from  the  soft 
palate  to  the  side  and  base  of  the  tongue  (palatoglossus  muscle), 
and  is  somewhat  prominent  as  it  passes  downward,  outward, 
and  forward.  The  posterior  pillar  approaches  more  closely  to 
its  fellow  of  the  opposite  side  than  does  the  anterior.  It  is 
formed  of  muscular  fibers  which  extend  from  the  soft  palate 
above  to  the  pharynx  below  (palatopharyngeus  muscle).  It  is 
somewhat  concave  in  its  downward  and  backward  course,  and 
while  closely  united  to  the  anterior  pillar  above,  is  separated 
from  it  below,  leaving  a  triangular  interval  or  niche  in  which  is 
lodged  a  small,  almond-shaped  body,  the  tonsil,  the  space  being 
known  as  the  tonsillar  recess.  The  intervening  space — bounded 
by  the  margins  of  the  soft  palate  above,  the  root  of  the  tongue 
below,  and  the  pillars  laterally — is  called  the  isthmus  of  the  fauces, 
and  establishes  the  communication  between  the  mouth  and  the 
pharynx.     The  free  margins  of  the  soft  palate,  assisted  by  the 


THE   INTERIOR   OF   THE   MOUTH.  37 

pillars  of  the  fauces,  mark  the  posterior  boundary  of  the  vumth. 
The  entire  surface  of  the  soft  palate  and  its  prolongations,  the 
pillars  of  the  fauces,  is  covered  with  mucous  membrane,  being 
continuous  with  that  of  the  mouth  on  its  anterior  surface,  and 
with  that  of  the  nasal  cavity  on  its  posterior  surface. 

Muscles  of  the  Soft  Palate. 

On  each  side  the  muscles  of  the  soft  palate  are  the  palato- 
glossus, palatopharymgeus,  levator  palati,  and  tensor  palati,  to- 
gether with  the  azygos  uvulce. 

Palatoglossus.  —  A  small  fasciculus  of  fibers,  somewhat 
cylindric  in  form,  expanding  at  either  end  into  a  thin  sheet.  It 
is  named  from  its  attachment  to  the  soft  palate  and  to  the 
tongue.  It  is  the  prominence  of  this  muscle,  together  with  its 
covering  of  mucous  membrane,  that  forms  the  anterior  pillar  of 
the  fauces. 

Origin. — By  a  thin  muscular  sheet  from  the  under  surface  of 
the  aponeurosis  of  the  soft  palate  near  the  median  line,  its  fibers 
uniting  with  those  of  the  opposite  side.  It  passes  downward  in 
front  of  the  tonsil  and  against  the  pharyngeal  wall. 

Insertion. — Into  the  side  and  base  of  the  tongue. 

Relations. — Superficially,  It  is  covered  by  the  mucous  mem- 
brane of  the  soft  palate  and  tongue  ;  deeply,  in  contact  with  the 
aponeurosis  of  the  soft  palate,  the  superior  constrictor  muscle 
of  the  pharynx,  and  one  of  the  muscles  of  the  tongue — the  hyo- 
glossus. 

Action. — The  lateral  walls  of  the  soft  palate  are  drawn  down, 
and  the  sides  of  the  tongue  are  drawn  upward  and  slightly 
backward.  Acting  in  conjunction  with  the  palatopharyngeus, 
the  opening  of  the  fauces  is  constricted. 

Palatopharyngeus. — This  muscle — also  named  from  its 
attachments — is  broad  above,  where  it  forms  the  greater  part 
of  the  lower  half  of  the  soft  palate.  Near  the  median  line  a  few 
of  its  fibers  blend  with  those  of  its  fellow  of  the  opposite  side. 

Origin. — By  two  heads  from  a  point  near  the  raphe  or  median 
line  of  the  soft  palate,  passing  downward  and  slighdy  backward, 
forming,  with  its  covering  of  mucous  membrane,  the  posterior 
pillar  of  the  fauces. 


3$  ANATOMY. 

I'dsertion. — Into  the  posterior  border  of  the  thyroid  cartilage, 
and  to  the  inner  surface  of  the  lower  part  of  the  pharynx. 

Relations. — In  the  soft  palate,  superficially,  with  the  mucous 
membrane,  both  anteriorly  and  posteriorly;  above,  with  the  leva- 
tor palati;  and  beneath,  by  the  mucous  glands.  In  the  posterior 
pillar  it  is  surrounded  with  mucous  membrane,  and  in  the 
pharynx  by  the  constrictor  muscles  of  the  pharynx  and  the 
mucous  membrane. 

Action. — To  constrict  the  opening  of  the  fauces,  by  bringing 
together  the  posterior  pillars,  thus  depressing  the  soft  palate 
and  elevating  the  pharynx.  It  controls  the  position  of  the  soft 
palate  during  respiration,  and  elevates  the  pharynx  during 
deglutition. 

Levator  Palati. — This  is  a  moderately  thick  muscle,  and 
derives  its  name  from  its  action  upon  the  soft  palate. 

Origin. — By  a  short  tendon  from  the  under  surface  of  the 
petrous  portion  of  the  temporal  bone,  and  from  the  posterior 
and  inferior  aspect  of  the  cartilage  of  the  Eustachian  tube. 

Insertion. — After  passing  downward  by  the  side  of  the  poste- 
rior nares  it  is  inserted  into  the  median  line  of  the  soft  palate, 
where  its  fibers  unite  with  those  of  its  fellow  of  the  opposite 
side. 

Relations. — Externally,  with  the  tensor  palati  and  superior 
constrictor  muscles  ;  internally  and  posteriorly,  with  the  mucous 
membrane. 

Action. — To  raise  the  soft  palate,  bringing  it  against  the  pos- 
terior wall  of  the  pharynx. 

Tensor  Palati. — This  is  a  slender  and  flattened  muscular 
sheet,  and  receives  its  name  from  its  action  upon  the  soft 
palate. 

Ojngin. — From  the  scaphoid  fossa  at  the  base  of  the  internal 
pterygoid  plate  and  the  spinous  process  of  the  sphenoid  bone  ; 
also  from  the  outer  side  of  the  anterior  aspect  of  the  Eustachian 
tube. 

Insertion. — After  descending  between  the  internal  pterygoid 
muscle  and  the  internal  pterj'goid  plate,  and  winding  around  the 
hamular  process  of  the  latter,  it  is  inserted  into  the  transverse 
ridge  on  the  horizontal  portion  of  the  palate  bone,  and  at  the 


THE    INTERIOR    OF    THE    MOUTH.  39 

median  line  of  the  soft  palate,  where  it  is  continuous  with  the 
aponeurosis  of  the  opposite  side. 

Action. — To  tighten  or  spread  the  soft  palate  laterally,  form- 
ing a  septum  between  the  posterior  nares  and  the  pharynx.  It 
also  opens  the  Eustachian  tube  during  deglutition. 

The  azygos  uvulae  (so  named  because  it  was  at  one  time 
supposed  to  be  a  single  muscle)  is  composed  of  a  pair  of  small 
muscles  which  oidgmate  from  the  aponeurosis  of  the  soft  palate 
and  the  nasal  spine  of  the  palate  bone.  They  pass  downward 
and  form  or  are  insej'ted  into  the  uvula. 

Relations. — Anteriorly,  with  the  levator  palati,  palatoglossi, 
and  a  part  of  the  palatopharyngei ;  posteriorly,  to  the  mucous 
membrane. 

Action. — To  shorten  or  draw  up  the  uvula. 

Blood-supply  to  the  Soft  Palate. 

The  poste^Hor  or  descending  palatal  bt'ancli  of  the  deep  facial 
artery,  after  emerging  from  the  posterior  palatal  canal,  sends  its 
posterior  division  backward  and  downward  to  the  soft  palate,  in 
the  substance  of  which  they  anastomose  with  the  ascending  palatal 
artery.  After  passing  over  the  superior  border  of  the  pharynx, 
the  ascendijig  pharyngeal  artery  sends  off  branches  which  are 
distributed  to  the  soft  palate.  A  few  branches  of  the  superior 
palatal  branch  of  the  internal  maxillary  and  a  few  twigs  from 
the  lingual  artery  also  convey  blood  to  the  parts. 

Course  of  the  Blood  from  the  Heart  to  the  Soft 
Palate. — From  the  heart  to  the  aorta,  to  the  common  carotid, 
to  the  external  carotid,  to  the  facial  or  lingual,  to  the  various 
branches  named  above,  to  the  soft  palate.  The  return  of  the 
blood  to  the  heart  is  principally  through  the  superior  and  infe- 
rior or  descending  palatal  veins,  both  of  which  closely  follow 
the  course  of  the  arteries  of  the  same  name. 

Nerves  of  the  Soft  Palate. — The  small,  or  posterior,  pala- 
tal, the  external  palatal  (both  of  which  are  branches  of  Meckel's 
ganglion),  branches  of  the  glosso-pharyngeal  nerve,  and  the 
following  nerves  which  supply  the  various  muscles :  filaments 
from  the  pharyngeal  plexus  to  the  palatoglossus  and  palato- 
pharyngeus,  branches  of  the  Vidian  to  the  levator  palati  and 


azygos   uvulae,  and   from   the  mandibular  division  of  the  fifth 
nerve  to  the  tensor  palati.'^' 

THE  INFERIOR  PORTION  OR  FLOOR  OF  THE   MOUTH  (Fig.  4). 

This  half  of  the  cavity  of  the  mouth  contains  the  tongue  and 

its  attached  muscles,  the  sixteen   lower  teeth  firmly  implanted 


Fig.  4. — The  Inferior  Portion  or  Floor  of  the  Mouth. 

in  the  jaw,  and  the  gums  covering-  the  alveolar  walls.  The  base 
or  osseous  framework  upon  which  this  portion  of  the  mouth  is 
constructed  is  principally  made  up  of  a  single  bone,  the  inferior 
maxillary,  mandible,  or  lower  jaw  bone  (see  Bones  of  the  Mouth, 
p.  51).    The  hyoid  bone,  situated  in  the  median  line  at  the  upper 

*  A  description  of  the  upper  teeth  will  be  found  in  another  chapler. 


THE    INTERIOR   OF   THE   MOUTH.  41 

part  of  the  neck,  and  at  the  base  of  the  tongue,  giving  attach- 
ment to  many  of  the  muscles  about  the  Hoor  of  the  mouth,  may 
also  be  considered  in  this  connection.  The  floor  of  the  mouth 
is  bounded  anteriorly  by  the  lower  lip,  laterally  by  the  cheeks, 
and  below  by  the  muscles  attached  to  the  external  and  internal 
oblique  lines  of  the  mandible. 

THE  TONGUE  (Fig.  5). 

The  tongue  is   a  freely  movable,  highly  sensitive,  muscular 


True  Vocal  Cord        Posteiior  Wall  of  Ihe  Pharynx         Corniculum  Laryng 


Fungiform  Papilla;       Circumvallale  Papillae 
Fig.  5. — Superior  Aperture  of  Larynx. — [Deava:) 

organ.      It  assists  in   the  function  of  speech,  participating,  also. 


in  the  special  sense  of  taste,  and  in  mastication  and  deglutition. 
The  organ  is  attached  posteriorly  to  a  U-shaped  bone,  the  hyoid 
bone,  which  of  itself  is  movable,  and  is  placed  in  the  neck  be- 
tween the  angles  of  the  lower  jaw  and  the  thyroid  cartilage. 

The  tongue  is  suspended  and  kept  in  its  position  in  the  mouth 
by  numerous  muscles,  some  of  which  are  attached  to  the  base 
of  the  skull,  and  others  to  the  lower  jaw  and  hyoid  bone. 

The  size  of  the  tongue  bears  little  or  no  relation  to  the 
size  of  the  individual,  but  is  proportioned  to  the  capacity  of  the 
alveolar  arch,  which  space  it  completely  fills  when  at  rest.  The 
shape  of  the  tongue  is  controlled  by  the  shape  of  the  alveolar 
arch  ;  thus,  when  the  arch  is  contracted,  narrow,  and  pointed,  the 
margins  of  the  tongue,  when  at  rest,  will  assume  that  form   [a, 

Fig.  6)  ;  but  when  the  arch  is 
broad  and  rounded  anteriorly, 
the  margins  of  the  tongue  will 
also  be  broad  and  rounded 
(b,  Fig.  6). 

The  substance  of  the  tongue 
is  chiefly  composed  of  muscular 
fibers,  which  are  arranged  in  a 
complicated  manner,  crossing 
one  another  at  various  angles, 
"'      Y\G  d       "  ^'\y\'s,  making  the  movements  of 

the  organ  exceedingly  varied 
and  extensive.  Fibrous,  areolar,  and  fatty  tissues  enter  into  its 
structure,  and  it  is  freely  supplied  with  blood-vessels  and  nerves. 
Its  free  surface  is  covered  by  sensitive  mucous  membrane,  and 
over  its  entire  surface  are  numerous  mucous  follicles  and  glands. 
Before  continuing  the  description  of  the  tongue,  it  will  be 
necessary  to  name  its  parts.  The  upper  surface,  or  that  facing 
the  roof  of  the  mouth,  is  the  dorsum;  those  portions  directed 
toward  the  cheeks  are  known  as  the  sides  of  the  tongue  ;  while 
the  hem  which  unites  these  two  borders  at  the  median  line,  and 
extending  a  short  distance  backward  on  either  side,  is  the  tip. 
That  portion  between  the  frenum  and  extending  back  to  the 
pillars  of  the  fauces  is  the  base,  while  that  part  of  the  dorsum 
immediately  posterior  to  the  tip  is  the  post-tip,  that  region  which 


THE   INTERIOR   OF   THE    MOUTH.  43 

lies  between  the  post-tip  and  the  base  being  \h<t  prebase.  The 
dorsum,  sides,  and  tip  are  free,  while  the  base  is  attached  by 
muscles  to  the  lower  jaw  and  hyoid  bone. 

From  the  base  to  the  epiglottis  is  a  fold  which  serves  to  limit 
the  movement  of  the  latter  organ,  and  from  the  sides  of  the 
base  the  pillars  of  the  fauces  are  given  off.  Under  the  anterior 
free  extremity  in  the  median  line  is  a  fibrous  muscular  lamina 
or  ligament,  the  fre^ium,  which  connects  this  part  of  the  organ 
with  the  lower  jaw  and  marks  the  anterior  border  of  the  base  of 
the  tongue.  The  tongue  is  divided  through  its  anterior  two- 
thirds  by  a  slight  longitudinal  furrow,  the  median  raphe,  which 
ends  posteriorly  near  a  small  foramen,  not  constant  in  the  adult 
tongue,  but  plainly  observed  in  the  fetus  or  infant,  the  foramen 
ccBCiim.  This  foramen  represents  the  upper  termination  of  the 
thyreoglossus  duct. 

Papillae  of  the  Tongue. — Over  the  anterior  two-thirds  ot 
the  dorsum  and  the  sides  and  tip  of  the  tongue  are  a  number 
of  small,  soft,  conic  eminences,  which  are  known  as  the  papillae 
of  the  tongue.  These  are  most  numerous  over  the  anterior  part 
of  the  dorsum,  and  at  the  back  they  are  covered  and  partly 
hidden  by  an  epithelial  coating.  In  general,  the  papillae  are 
quite  similar  to  those  of  the  integument,  not  being  compound 
organs  in  their  vascular  and  nervous  supply.  In  consequence 
of  their  variation  in  form,  the  papillae  are  variously  named.  The 
largest  papillae,  being  arranged  like  the  letter  V,  are  called  the 
circunivallate  or  calciform  ;  those  of  medium  size,  \h&  fungiform^ 
are  so  named  from  their  resemblance  to  a  young  mushroom  ; 
and  the  smallest  and  most  numerous  are  known  as  the  conic 
or  filiform  papillae.  Each  papilla  presents  a  broad,  free  end, 
and  is  attached  by  a  constricted  base,  which  rests  in  a  small,  cup- 
like concavity,  about  the  margins  of  which  is  a  well-formed 
circular  rim.  Beneath  the  thick  epithelium  of  these  parts  are 
numerous  secondary  papillae,  and  about  the  base  of  each  papilla 
are  the  openings  of  one  or  more  glands. 

The  circuvrjallate  or  calciform  papillae  (Fig.  5)  form  a  V-shaped 
line  at  the  posterior  boundary  of  the  dorsum.  They  are  few  in 
number  (varying  from  six  to  twelve),  but  are  largest  in  size, 
not  infrequently  measuring  y^  of  an  inch  in  diameter.     These 


papillae  are  generally  regarded  as  being  gustatory,  or  directly 
interested  in  the  sense  of  taste.  Each  papilla  is  capped  with  a 
small  secondary  papilla. 

'X\\it  fungiform  papilla;  (Fig.  5),  of  medium  size,  varying  from 
2V  to  Jq  of  an  inch  in  diameter,  are  scattered  over  the  dorsum, 
sides,  and  tip  of  the  tongue  at  irregular  intervals,  and  are  much 
more  highly  colored  than  the  smaller  papillae  which  surround  them. 
They  vary  greatly  in  number,  and  being  principally  gustatory, 
account  in  a  great  measure  for  the  diversity  in  the  acuteness 
of  the  sense  of  taste  in  different  individuals.  These  papillae, 
like  the  circumvallate,  are  capped  with  smaller  secondary  papillae. 

The  conic  or  filiform  papillae  (Fig.  5)  are  the  smallest  and 
most  numerous,  and  are  thickly  scattered  over  the  entire  surface 
of  the  dorsum  in  front  of  the  circumvallate  as  well  as  over  the 
sides  and  tip  of  the  tongue.  They  are  placed  closely  together, 
and  with  such  regularity  that  they  fairly  ridge  the  tongue  with 
delicate  lines,  which  run  parallel  with  the  circumvallate  in  that 
region,  but  as  the  tip  is  approached  they  become  transversely 
inclined.  These  papillae  are  generally  regarded  as  being  tactile 
or  directly  interested  in  the  sense  of  touch,  and  are  concerned  in 
directing  the  movements  of  the  food  during  mastication.  They 
also  possess  secondary  papillae  upon  their  surfaces. 

Immediately  posterior  to  the  circumvallate  papillae  are  two 
shallow  grooves  which  follow  the  V-shaped  line  of  the  papillae 
and  unite  at  the  foramen  caecum.  These  grooves  serve  to  indi- 
cate the  line  of  junction  between  the  anterior  and  posterior 
portions  of  the  tongue.  The  latter  not  being  within  the  cavity 
of  the  mouth  will  not  be  described. 

Muscles  of  the  Tongue   (Fig.  7). 

The  muscles  of  the  tongue  are  both  c xtrinsic-oiitward  or  e.x- 
ternal,  and  intrinsic-inherent,  inward,  or  special. 

The  extrinsic  muscles  include  those  which  have  their  origin 
from  the  base  of  the  skull,  the  hyoid  bone,  or  the  mandible,  and 
are  the  Jiyoglossus,  geniohyogiossus,  styloglossus,  palatoglossus, 
and  a  few  fibers  of  the  superior  constrictor  of  the  pharynx. 
The  intrinsic  muscles  which  make  up  the  bulk  of  the  tongue  are 
two  in  number,  the  superior  lingualis  and  inferior  bngualis. 


THE   INTERIOR   OF   THE   MOUTH.  45 

Hyoglossus. — As  its  name  implies,  this  muscle  extends  from 
the  hyoid  bone  to  the  tongue.  Its  fibers  are  so  arranged  that 
they  form  a  thin  square  sheet. 

Origin. — It  arises  from  the  whole  length  of  the  upper  border 
of  the  great  cornu,  from  the  body,  and  by  a  few  fibers  from  the 


DORSUM  OF  TONGUE 


Genio-hyo-gl 
Genio  hvoid 


GREATER  CORNU  OF  HYOID  BONE 


STYLOID  PROCESS 
Stylo-hyoid 


Stylo-pharyngeus 


CARTILAGO  TRITICEA 


THYROID  CARTILAGE 


Median  portion  of 
crico-thyroid 
membrane 

CRICOID  CARTILAGE 

FIRST  RING  OF  TRACHEA 


Fig.  7. — Side  View  of  the  Tongue,  with  its  Muscles. — [Morris.) 


lesser  cornu  of  the  hyoid  bone.  At  their  point  of  origin  the 
fibers  are  in  the  form  of  a  thin  sheet,  and  ascend  toward  the 
tongue  almost  parallel  to  one  another,  but  before  reaching  the 
tongue  the  anterior  fibers  pass  slightly  forward,  and  at  the  upper 
margin  of  the  side  of  the  tongue  bend  inward  and  join  the  fibers 
of  the  superior  lingualis.      In  their  distribution  to  this  part  of 


46  ANATOMY. 

the  tongue  they  form   a  kind  of   submucous  covering  to  the 
organ. 

Insertion. — Into  the  posterior  half  of  the  side  of  the  tongue, 
between  the  styloglossus  and  superior  lingualls  muscles. 

Relations. — Externally,  with  the  digastricus,  styloglossus, 
stylohyoid,  and  mylohyoid  muscles,  the  lingual  and  hypoglossal 
nerves,  Wharton's  duct,  and  the  sublingual  gland.  Internally, 
with  the  lingualis,  geniohyoglossus,  and  middle  constrictor  of 
the  pharynx  muscles,  the  lingual  artery,  and  the  glossopharyn- 
geal nerve. 

Action. — To  extend  the  tongue  and  to  draw  it  backward,  also 
to  draw  downward  the  sides  of  the  tongue,  making  its  dorsum 
more  convex  transversely. 

Geniohyoglossus  (Fig.  7). — This  muscle  also  receives  its 
name  from  its  three  points  of  attachment,  the  chin  internally,  the 
hyold  bone,  and  the  tongue.  It  is  a  triangular-shaped  muscle, 
narrow  and  pointed  at  its  attachment  to  the  mandible,  and  broad 
and  fan-shaped  on  approaching  the  tongue.  Being  near  the 
median  line,  it  is  separated  from  its  fellow  of  the  opposite  side 
by  a  thin  layer  of  connective  tissue,  the  septum  of  the  tongue. 

Origin. — It  arises  by  a  short  tendon  from  the  upper  genial 
tubercle  of  the  lower  jaw,  from  which  point  its  fleshy  fibers 
diverge  fan-like  to  its  extensive  insertion. 

Insertion. — To  the  whole  length  of  the  tongue  from  base  to 
apex  immediately  external  to  the  median  line,  into  the  body  of 
the  hyoid  bone,  and  by  a  few  fibers  into  the  side  of  the  pharynx. 

Relations. — By  its  inner  surface,  with  the  septum  of  the  tongue 
and  its  fellow  of  the  opposite  side;  by  its  outer  surface,  with  the 
hyoglossus,  mylohyoides,  styloglossus,  and  lingualis  muscles, 
sublingual  gland,  lingual  artery,  and  hypoglossal  nerve.  Su- 
periorly, with  the  mucous  membrane  of  the  floor  of  the  mouth  ; 
inferiorly,  with  the  geniohyoid  muscle. 

Action. — Its  anterior  fibers  assist  in  drawing  back  the  tip  of 
the  tongue,  its  posterior  fibers  throwing  forward  and  protruding 
the  tongue.  This  muscle  also  depresses  the  center  of  the  dor- 
sum longitudinally,  making  it  concave  transversely,  and  some  of 
the  lower  fibers  which  are  attached  to  the  hyoid  bone  elevate 
the  bone  and  assist  in  raising  the  tongue. 


THE    INTERIOR    OF   THE    MOUTH.  47 

Styloglossus. — Also  named  from  its  attachment,  is  a  long 
fan-shaped  muscle,  somewhat  compressed  laterally. 

Origin. — From  its  point  of  origin  at  the  tip  of  the  styloid 
process  of  the  temporal  bone,  and  from  a  portion  of  the  stylo- 
maxillary  ligament,  it  passes  with  a  long  curve,  forward,  slightly 
downward,  then  upward  and  inward  to  its  place  of  insertion  at 
the  side  of  the  tonojue. 


Vena  Conies  ^ 

1  Ranine  Arteries    \'eiia  Comes 

Intrinsic  Muscular  Fibers 

Fig.  8. — Transverse  Section  of  One-half  of  Tongue. — {Denver.) 


Insertion. — Upon  reaching  the  side  of  the  tongue  it  divides 
into  two  portions,  the  fibers  of  one  portion  passing  transversely 
inward,  while  the  others  pass  longitudinally  along  the  side  of 
the  tongue. 

Relations. — Externally,  with  the  internal  pterygoid  muscle, 
parotid  and  sublingual    glands,  lineal    nerve,  and  the  mucous 


48  ANATOMY. 

membrane  of  the  mouth ;  internally,  with  the  superior  con- 
strictor and  hyoglossus  muscles,  and  with  the  tonsil. 

Action. — To  draw  the  tongue  backward  and  to  produce  a 
transverse  concavity  to  its  upper  surface  by  elevating  its  sides. 

Superior  Lingualis. — This  is  one  of  the  intrinsic  muscles, 
and  is  situated  immediately  beneath  the  mucous  membrane,  ex- 
tending from  the  base  to  the  tip  of  the  organ. 


FR/ENUM  LIN3'J,€ 


Lingualis  inferior 


Mylo-hyoid,  reflected 

Steruo-hyoid    — ' — ^ 

Fig.  9. — Under  Surface  of  the  Tongue  with  Muscles. — [Morris.' 


Inferior  Lingualis  (Fig.  9). — This  muscle  is  placed  near  the 
under  surface  of  the  tongue,  and  is  composed  of  two  bands  which 
extend  from  base  to  apex,  some  of  its  fibers  being  attached  pos- 
teriorly to  the  hyoid  bone,  and  in  passing  forward  are  placed 
between  the  hyoglossus  and  geniohyoglossus.  Anteriorly,  its 
fibers  blend  with  those  of  the  styloglossus. 

Many  of  the  fibers  of  this  muscle  run  transversely  and  are 


THE   INTERIOR   OF   THE   MOUTH.  49 

placed  between  the  two  former  intrinsic  muscles.  These,  to- 
gether with  some  fatty  tissue,  compose  the  greater  part  of  the 
substance  of  the  tongue.  The  fibers  are  attached  at  the  median 
line  to  the  fibrocartilaginous  septum  of  the  tongue,  and  laterally 
to  the  mucous  membrane.  In  connection  with  the  transverse 
fibers  there  are  a  few  placed  vertically,  which  pass  by  long  curves 
from  the  dorsum  to  the  under  surface  of  the  tongue. 

Blood-vessels  of  the  Tongue  (Fig.  lo). 

This  organ  receives  its  blood  principally  through  the  lingzial, 
facial,  and  ascending  pharyngeal  arteries.     The  lingual  artery 


-Descending  palaiijie  A . 

-Anferior  br,  of  descending  palatine  A . 
,.  Styto-ylossusM. 

yPatafo  -(^iossusAf. 

^  Tonsillar  br.dorsalis  linguae  A . 


Posterior  br.  of 
descending  palatine  A 

Palatine  br.  of 
a-s  cen  dinqphaijn'qeal 

Ascending  phan.'nqealA'- 

AscendJnq  palatine  hr 
'of facial  A 
Tonsillar    br — 
of  fcLcial  A 
Stylo-phaiynqeus  M.- 

Facial  A. 

Middle  conslHcforM .- 
Dorsalis  linguae  A 

Lingual  A- 
ExleiTtal  carotid  A 


Superior  ihyroid 

infra-  iiyoid    br  oj skp.tliyroid 

^"P''%l'yiZguaiA.  Sty-lo-hyoidM.     ^Sublingual  a: 

Fig.   10. — Arteries  of  Tongue  and  Tonsil. — [Dearer.) 


arises  from  the  front  of  the  external  carotid  near  the  facial,  and 
often  as  a  common  trunk  with  it.  From  its  point  of  origin  to 
the  tongue  it  is  divided  into  three  portions,  the  first  or  oblique, 
the  second  or  horizontal,  and  the  third  or  ascending,  and  it  is 
this  latter  portion  which  directly  supplies  the  tongue.  Ascending 
tortuously  beneath  the  hyoglossus  muscle,  it  reaches  the  under 
surface  of  the  tongue,  and,  lying  between  the  lingualis  and  hyo- 
4 


50  ANATOMY. 

glossus  muscles,  it  is  continued  to  the  under  surface  of  the  tip  of 
the  tongue,  at  which  point  it  is  called  the  ranine  artery.  At  a 
point  about  corresponding  with  the  posterior  margin  of  the  hyo- 
glossus  muscle  a  branch  is  given  off  (the  dorsalis  liiigiics),  which 
passes  almost  directly  upward,  and,  after  dividing  into  two  or 
more  small  branches,  supplies  the  back  part  of  the  dorsum  of 
the  tongue  and  the  mucous  membrane  about  the  circumvallate 
papillae.  At  the  anterior  border  of  the  hyoglossus  muscle 
another  branch  is  given  off  (the  sublingual  artery)  supplying 
the  anterior  muscular  structure  of  the  floor  of  the  mouth.  The 
facial  artery  by  one  of  its  muscular  branches  supplies  the  stylo- 
glossus muscle. 

Course  of  the  Blood  From  the  Heart  to  the  Tongue. — 
From  the  heart  to  the  aorta,  to  the  common  carotid,  to  the  ex- 
ternal carotid,  to  the  lingual  artery,  and  its  smaller  branches  to 
the  tongue.  From  the  tongue  the  blood  is  returned  to  the 
heart  principally  through  the  lingual  vein,  which  begins  at  the 
ranine  vein  beneath  the  tip  of  the  tongue,  passes  backward  under 
cover  of  the  mucous  membrane,  following  the  course  of  the 
lingual  artery  until  the  hyoglossus  muscle  is  reached,  beyond 
which  point  the  fibers  of  the  muscle  separate  the  artery  from 
the  vein.  After  receiving  the  sublingual  and  dorsalis  linguae 
veins,  the  course  of  which  corresponds  to  the  arteries  of  the 
same  name,  the  vein  passes  backward  and  downward  and 
empties  into  the  internal  jugular. 

Nerves  of  the  Tongue. 

The  mandibular  division  of  the  fifth  nerve  by  its  lingual 
branch  supplies  the  papillae  of  the  anterior  portion  and  sides  of 
the  tongue,  while  the  lingual  branch  of  the  glossopharyngeal 
supplies  the  circumvallate  papillae,  the  base,  and  posterior  sides. 
A  few  branches  of  the  superior  laryngeal  are  distributed  to  the 
back  part  of  the  root  of  the  organ.  The  motor  nerve  of  the 
tongue  is  the  hypoglossal  or  nintli,  supph'ing  both  the  extrinsic 
and  intrinsic  muscles.'^' 

■*  A  description  of  die  lower  teelli  will  be  found  in  another  chapter. 


CHAPTER    II. 

THE  BONES  OF  THE  MOUTH:  THE  SUPERIOR  MAXILL/E,  THE  PALATE 
BONES,  THE  INFERIOR  MAXILLA  OR  MANDIBLE. 

SUPERIOR  MAXILLARY  BONES. 

The  superior  maxillary  bones,  two  in  number,  one  on  each 
side  of  the  median  hne  or  center  of  the  face,  are  irregular  in 
shape,  and  may  be  classed  as  the  largest  bones  of  the  face,  with 
the  probable  exception  of  the  mandible  or  inferior  maxilla. 
From  the  central  position  which  they  occupy  they  contribute 
largely  to  the  bony  framework  of  this  portion  of  the  skull.  They 
are  not  only  instrumental  in  forming  the  major  portion  of  the 
roof  of  the  mouth  or  hard  palate,  but  assist  in  the  formation  of 
the  floor  of  the  orbit,  and  the  sides  and  base  of  the  nasal  cham- 
ber. They  furnish  a  solid  and  firm  foundation  for  the  sixteen 
upper  teeth,  and  by  their  variety  in  form  contribute  much  to 
the  character  and  quality  of  the  voice.  The  outer  or  facial 
surface  of  these  bones  provides  attachment  for  numerous  mus- 
cles. Each  superior  maxillary  bone  presents  for  examination  a 
body,  four  surfaces,  and  four  processes.  The  body  may  be 
described  as  forming  an  irregular  triangle,  its  general  contour 
depending  much  upon  the  temperament  of  the  subject  and  con- 
sequent character  of  the  teeth.  Within  the  body  of  the  bone  is 
an  irregular  cavity,  the  maxillary  sinus  or  antrum  of  Highmore. 

The  four  surfaces  of  the  body  of  the  bone  are  the  superior  or 
orbital,  the  lateral  or  facial,  the  proximal  or  nasal,  and  the  pos- 
terior or  zygomatic. 

The  Superior  or  Orbital  Surface,  which  assists  in  forming 
the  greater  portion  of  the  floor  of  the  orbit,  is  slighdy  concave 
over  its  anterior  two-thirds,  and  somewhat  convex  over  the  re- 
maining or  posterior  third.  The  three  borders  of  this  surface 
form  almost  an  equilateral  triangle,  and  are  named,  as  indicated 
by  their  location,  the  anterior,  the  posterior,  and  the  mesial  or 
proximal.     The  anterior  border  is  convex  from  before  backward 

51 


and  slightly  concave  throughout  its  length.  That  portion  which 
forms  a  part  of  the  lower  border  of  the  completed  orbit  is  smooth, 
while  the  remaining  portion  is  roughened  to  form  an  articulation 
with  the  malar  bone.  The  posterior''  border  extends  from  the 
center  of  the  malar  process  backward  and  inward  to  the  orbital 
process  of  the  palate  bone,  which  articulates  with  the  superior 
maxillary  at  this  point.  A  portion  of  this  border,  together  M'ith 
the  orbital  process  of  the  palate  bone,  is  instrumental  in  forming 
the  anterior  boundary  of  the  sphenomaxillary  fissure. 

The  mesial  or  proximal  border  is  marked  by  an  irregular 
thin  edge,  which  articulates  with  a  portion  of  two  bones,  the 
lacrymal  anteriorly,  and  the  os  planum  of  the  ethmoid  bone 
posteriorly.  Only  the  posterior  two-thirds  of  this  border  pre- 
sents an  articulating  edge,  the  remaining  or  anterior  third  being 
smooth  and  forming  the  commencement  of  the  lacrymal  groove, 
which  in  the  articulated  skull  becomes  a  canal,  passing  down- 
ward and  backward  to  communicate  with  the  inferior  meatus 
of  the  nose.  Beginning  at  the  posterior  border  of  this  sur- 
face and  running  forward  will  be  found  a  deep  groove — the 
infra-orbital  groove.  When  near  the  center  of  the  surface,  this 
groove  dips  down  and  is  covered  by  a  layer  of  bone,  from  which 
point  it  passes  forward  as  a  canal, — the  infra-orbital  canal, — 
making  its  exit  at  a  point  about  yi  of  an  inch  below  the  border 
of  the  orbit,  near  the  center  of  the  facial  surface  of  the  bone, 
the  foramen  thus  formed  being  the  infra-orbital  foramen.  Near 
the  root  of  the  nasal  process,  and  immediately  within  the  ante- 
rior border  of  this  surface,  is  a  small  depression  which  marks 
the  origin  of  the  inferior  oblique  muscle  of  the  eyeball. 

The  Lateral  or  Facial  Surface  (Fig.  ii). — This  surface  is 
made  up  of  the  anterior  part  of  the  bone  ;  it  is  irregularly  con- 
cave, and  presents  a  greater  variety  in  form  than  any  other  part 
of  the  bone,  with  the  single  exception  of  the  palatal  process.  It 
is  bounded  above  by  the  infra-orbital  ridge,  and  the  roughened 
surface  of  the  malar  process  which  articulates  with  the  malar 
bone  ;  below,  by  the  border  of  the  alveolar  process  ;  anteriorly, 
by  the  frail  concave  border  of  the  opening  into  the  nasal  cavity, 
the  anterior  nasal  spine,  and  the  perpendicular  margins  of  the 
bone  beneath.     Posteriorly,  this  surface  is  separated  from  the 


SUPERIOR   MAXILLARY   BONES. 


S3 


posterior  or  zygomatic  surface  by  a  strong  projecting  eminence, 
the  malar  process. 

The  canine  fossa  is  a  deep  depression,  situated  almost  in  the 
center  of  this  surface,  the  bone  at  this  point  being  extremely 
thin  and  closely  related  to  the  floor  of  the  antrum.  The  concave 
floor  of  this  fossa  is  frequently  traversed  by  one  or  two  smaller 
convex  ridges,  corresponding  to  the  roots  of  the  bicuspid  teeth. 

The  canine  eminence  is  a  prominent  ridge  running  vertical  to 


Nasal  Process 


Orbital  Surface 


Malar  Process 


Tuberosity 


Fig.   It. — Left  Superior  Maxilla,  Outer  or  Facial  Surface. 


the  body  of  the  bone  immediately  anterior  to  the  canine  fossa, 
and  corresponding  in  position  to  the  root  of  the  cuspid  tooth, 
the  size  and  type  of  the  tooth  having  much  to  do  with  its  extent 
and  prominence.  This  ridge  gives  origin  to  one  of  the  depressor 
muscles  of  the  upper  lip,  and  also  to  one  of  the  depressor 
muscles  of  the  wing  of  the  nose.  The  incisive  or  viyrtiform 
fossa  is  a  depression  found  between  the  canine  eminence  and  the 
inner  margin  of  the  bone.     The  depth  of  this  tossa  is  in  a  meas- 


54  ANATOMY. 

lire  controlled  by  the  position  and  size  of  the  teeth,  and  by  the 
amount  of  prominence  in  the  canine  eminence. 

The  infra-orbital  forameji,  which  transmits  the  infra-orbital 
nerves  and  blood-vessels,  is  immediately  below  the  center  of  the 
infra-orbital  ridge,  and  near  the  upper  margin  of  the  canine 
fossa.  It  is  oval  in  form  and  faces  almost  directly  toward  the 
median  line.  Between  this  foramen  and  the  infra-orbital  ridge 
is  the  point  of  origin  for  the  principal  levator  muscle  of  the  upper 
lip,  the  levator  labii  superioris  proprius.  The  whole  extent  of 
the  facial  surface  may  present  a  number  of  vertical  ridges,  or 
the  same  space  may  be  regular  and  smooth,  the  condition  being 
controlled  by  the  size  and  shape  of  the  tooth-roots  and  the 
thickness  of  the  bone  covering  them.  One  of  the  levator 
muscles  of  the  angle  of  the  mouth,  the  levator  anguli  oris,  is 
attached  to  this  surface  near  the  upper  border  of  the  canine 
fossa. 

The  Proximal  or  Nasal  Surface  (Fig.  12). — Above,  this 
surface  presents  a  large,  irregular  opening  into  the  maxillary 
sinus,  this  opening  being  nearly  closed  in  the  articulated  skull 
by  neighboring  bones.  In  front  of  the  opening  into  the  sinus, 
and  standing  perpendicular  from  the  body  of  the  bone,  is  the 
strong  ascending  plate  of  the  nasal  process,  marked  near  its 
lower  extremity  by  a  rough,  horizontal  ridge,  the  inferior  tur- 
binated crest,  which  gives  attachment  to  the  inferior  turbinated 
bone.  The  smooth,  concave  surface  immediately  above  this 
ridge  corresponds  to  the  middle  meatus  of  the  nose,  and  forms 
the  external  wall  of  that  passage.  Below  the  opening  into  the 
sinus  and  the  nasal  process,  and  occupying  the  anterior  two- 
thirds  of  the  middle  of  this  surface,  is  a  large  semicircular  space, 
forming  the  outer  wall  of  the  inferior  meatus  of  the  nose. 
Below  this  space,  and  projecting  inward  from  the  body  of  the 
bone,  is  the  palatal  process,  which  articulates  with  the  correspond- 
ing process  of  the  opposite  bone.  At  the  anterior  superior 
angle  of  the  nasal  surface,  and  passing  downward  just  behind 
the  nasal  process,  is  the  lacrymal  groove.  In  the  articulated 
skull  this  groove  becomes  a  canal,  the  lacrymal  canal  the 
ethmoid  and  the  inferior  turbinated  bones  assisting  in  its  forma- 
tion.    The  canal  passes  downward  and  slightly  backward,  and 


SUPERIOR   MAXILLARY    BONES.  55 

opens  into  the  inferior  meatus  of  the  nose.  It  is  aioout  ^4  of  an 
inch  in  length,  and  gives  passage  to  the  lacrymonasal  duct. 

T/ie  lacrymal  tubercle  is  a  small  prominence  of  bone  formed 
at  the  junction  of  the  anterior  border  of  this  surface,  with  the 
external  surface  of  the  nasal  spine.  The  extended  portion  of 
the  lacrymal  duct,  the  lacrymal  sac,  finds  lodgment  at  this  point. 

Tlie  posterior  palatine  or  palatomaxillary  canal  commences 


Inferior  Meat 


Palale  Process 


Nasal  Process 


Middle  Meatus 

Tlni7i  plate  of 
hone  over  Lac- 
rymal Groove 


Fig.   12. — Left  Superior  Maxilla,  Internal,  Proximal,  or  Nasal  Surface. 

near  the  middle  of  the  posterior  border  of  this  surface,  appear- 
ing in  the  disarticulated  bone  as  a  groove,  and,  passing  downward 
and  forward,  gives  passage  to  the  posterior  palatine  vessels  and 
anterior  palatine  nerves.  The  canal  is  made  complete  by  the 
articulation  of  the  superior  maxillary  with  the  vertical  plate  of 
the  palate  bone.  On  the  posterior  portion  of  the  nasal  surface, 
extending  from  the  irregular  opening  into  the  antrum  downward 


S6  ANATOMY. 

to  a  point  opposite  the  palatal  process,  is  a  roughened  surface 
about  i^  of  an  inch  in  width,  which  marks  the  extent  of  articu- 
lation with  the  palate  bone. 

The  proximal  or  7iasal  sicrf ace  presents  four  borders — superior, 
inferior,  anterior,  and  posterior.  The  superior  border  is  irregular, 
and  articulates  with  the  lacrymal  and  ethmoid  bones.  The  inferior 
border  projects  inward,  and  forms  a  strong  horizontal  plate — the 
palatal  process.  This  process  defines  the  border  from  before 
backward  to  the  posterior  third,  at  which  point  it  is  marked  by 
the  lower  border  of  the  roughened  surface  which  articulates  with 
the  palate  bone.  The  anterior  border  is  sharp,  frail,  and  irreg- 
ular in  outline,  and  forms  the  free  margin  of  the  opening  into 
the  nasal  cavity.  The  posterior  border  is  marked  by  the  inner 
margin  of  the  zygomatic  surface,  being  smooth  upon  its  upper 
half,  and  roughened  upon  its  lower  half,  at  which  point  it  articu- 
lates with  the  palate  bone. 

The  Posterior  or  Zygomatic  Surface  (Fig.  1 1 ). — This  sur- 
face is  partly  convex  and  partly  concave,  and  is  bounded  above 
by  a  well-defined  margin,  which  serves  as  the  dividing-line  be-i 
tvveen  this  and  the  superior  or  orbital  surface.  This  border  is 
also  marked  by  a  roughened  margin  on  the  posterior  portion  of 
the  malar  process,  the  orbital  pordon  of  the  palate  bone  articu- 
lating at  this  point.  The  major  portion  of  this  border  is  smooth 
and  rounded,  forming  the  lower  border  of  the  sphenomaxillary 
fissure,  and  marked  by  a  notch,  the  commencement  of  the  infra- 
orbital groove.  The  outer  border  of  the  surface  is  formed  by 
the  malar  process,  and  by  a  line  drawn  from  this  point  directly 
downward  to  the  alveolar  process.  The  inner  border  is  smooth 
and  somewhat  irregular  above,  while  below  it  is  roughened  for 
articulation  with  the  palate  bone. 

The  htberosity,  which  also  forms  a  portion  of  the  inferior 
border  of  this  surface,  is  a  roughened  and  rounded  eminence  ot 
bone,  and  is  penetrated  by  a  number  of  nutrient  vessels,  which 
enter  the  many  small  foramina  at  this  point.  Between  the 
tuberosity  and  the  body  of  the  zygomatic  surface  are  several 
large  apertures  leading  into  canals,  which  pass  into,  and  give 
nourishment  to,  the  substance  of  the  bone.  These  canals  trans- 
mit the  posterior  dental  blood-vessels  and  nerves,  one  of  which, 


SUPERIOR   MAXILLARY    BONES.  57 

after  passing  over  the  outer  wall  of  the  maxillary  sinus,  unites 
with  the  anterior  dental  canal.  The  tuberosity  is  posterior  to, 
and  above,  the  third  molar  tooth,  in  some  instances  extending 
directly  backward  from  this  tooth  for  the  distance  of  half  an  inch 
or  more,  but  usually  the  tooth  penetrates  the  base  of  the  tuber- 
osity, leaving  but  a  thin  layer  of  bone  posterior  to  it. 

The  inferior  border  ol  the  posterior  or  zygomatic  surface  is 
formed  by  that  portion  of  the  alveolar  process  which  supports 
the  second  and  third  molar  teeth. 

The  bone  presents  four  processes  for  examination — the  nasal, 
the  malar,  the  palatal,  and  the  alveolar. 

The  nasal  process  is  a  strong,  irregular  piece  of  bone,  stand- 
ing vertically  above  the  body  of  the  bone  proper,  and  forming  the 
lateral  boundary  of  the  nose.  This  process  is  greatly  increased 
in.  strength  by  the  infra-orbital  ridge  joining  it  at  or  near  its 
base,  and  ascending  its  external  anterior  surface  to  some  extent. 
That  portion  of  the  process  posterior  to  its  junction  with  the 
infra-orbital  ridge  assists  in  forming  the  inner  wall  of  the  orbit. 

The  external  or  ante7-ior  surface  of  the  nasal  process  is  marked 
by  a  number  of  shallow  grooves,  traces  of  the  development  of 
the  bone.  Scattered  over  this  surface  are  a  number  of  small 
foramina,  the  entrances  to  minute  canals  transmitting  nutrient 
vessels  to  the  body  of  the  bone.  This  surface  gives  origin  to 
one  of  the  lip  muscles,  the  levator  labii  superioris  alaeque  nasi. 

Tlie  ijiternal  surface  of  the  nasal  process  is  usually  described 
as  including  all  that  portion  between  the  superior  border  and 
the  floor  of  the  anterior  nares.  The  surface  is  marked  by  two 
concave  portions  and  two  ridges.  The  two  ridges  divide  the 
surface  into  three  parts — the  superior  meatus,  the  middle  meatus, 
and  the  inferior  meatus  of  the  nose.  The  supenor  meatus  is  the 
smallest  of  the  three,  and  occupies  the  slightly  concave  space 
above  the  superior  ridge.  The  middle  meatus,  partly  concave, 
and  partly  convex,  includes  the  space  between  the  superior  and 
the  inferior  ridges,  and  extends  from  the  free  margin  of  the 
bone  in  front  to  the  lacrymal  groove  behind.  The  inferior 
meattcs,  which  is  much  the  largest,  occupies  all  that  concave 
surface  between  the  inferior  ridge  above  and  the  palatal  process 
below,  and  extends  from  the  anterior  margin  of  the  bone  back- 


58  ANATOMY. 

ward  to  the  point  of  articulation  with  the  palate  bone.  The  two 
ridges  previously  referred  to  are  known  as  the  supeinor  tur- 
binated C7'est,  which  articulates  with  the  middle  turbinated  bone> 
and  the  inferior  turbinated  crest,  which  articulates  with  the 
inferior  turbinated  bone. 

The  malar  process  is  a  large,  irregular  portion  of  bone 
situated  at  the  angle  of  separation  between  the  facial  and  zygo- 
matic surfaces,  and  presents  a  triangular,  roughened  surface  for 
articulation  with  the  malar  bone.  The  superior  boundary  of 
this  process  is  formed  by  the  orbital  surface  and  the  outer  end 
of  the  infra-orbital  ridge  ;  the  inferior  boundary  may  be  marked 
by  an  irregular  imaginary  line  running  from  the  upper  margin 
of  the  canine  fossa  to  a  point  between  the  first  and  second 
molar  teeth,  while  the  posterior  inferior  boundary  may  be  traced 
from  the  outer  superior  angle  of  the  zygomatic  surface  down- 
ward and  forward  to  the  point  above  referred  to.  This  process, 
as  well  as  the  nasal  process,  is  subject  to  much  variety  in  form 
and  general  outline.  The  malar  process,  assisting  as  it  does  in 
forming  what  is  commonly  called  the  cheek  bone,  is  particularly 
variable  in  size,  and  in  certain  types  and  races  it  is  so  prominent 
as  to  become  a  controlling  feature  in  the  facial  form.  One  of 
the  muscles  of  mastication — the  masseter — has  a  portion  of  its 
origin  from  the  malar  process. 

The  palatal  process  is  more  directly  interested  In  the  forma- 
tion of  the  cavity  of  the  mouth  than  any  other  portion  of  the 
superior  maxillary  bone.  By  articulating  with  its  fellow  of  the 
opposite  side,  It  forms  about  three-fourths  of  the  hard  palate  or 
roof  of  the  mouth,  the  remaining  fourth  being  formed  by  a  por- 
tion of  the  palate  bones.  It  Is  thick  and  strong,  and  projects 
horizontally  Inward  from  the  Inner  surface  of  the  body  of  the 
bone.  It  presents  two  surfaces  for  examination, — a  superior  or 
nasal  surface,  and  an  Inferior  or  oral  surface.  'Wi^  superior  or 
nasal  surface  Is  smooth  and  more  or  less  concave,  and  forms  the 
floor  of  the  nares.  The  inferior  or  oral  S7irfacc  is  also  concave, 
but  Is  much  roughened  by  numerous  small  projections,  between 
which  are  lodged  the  mucous  glands.  Upon  the  anterior  por- 
tion of  this  surface  are  a  number  of  small  foramina,  which  mark 
the  entrance  to  numerous  small  canals  giving  passage  to  nutrient 


SUPERIOR    MAXILLARY   BONES.  59 

vessels  to  supply  the  body  of  the  bone.  Near  the  center  of  the 
posterior  third  are  the  anteroposterior  grooves,  which  accommo- 
date the  posterior  palatine  nerves  and  blood-vessels.  This 
process  also  presents  for  examination  three  borders  and  various 
other  points  of  interest.  The  three  borders  are  the  anterior, 
posterior,  and  mesial.  The  anterior  border  is  thick  and  some- 
what irregular  ;  the  posterior  border  is  thin  and  frail,  and  articu- 
lates with  a  portion  of  the  palate  bone.  The  mesial  border 
presents  a  wide  articulating  surface  in  front,  behind  it  is  narrow, 
the  whole  extent  of  this  border  articulating  with  the  correspond- 
ing process  of  the  opposite  bone. 

The  Nasal  Spine. — At  the  anterior  superior  angle  of  the 
palatal  process  is  a  well-defined  spine, — the  nasal  spine, — being 
formed  by  a  prolongation  of  the  process  beyond  the  level  of  the 
facial  surface  of  the  bone.  This  process,  when  articulated  with 
its  fellow  of  the  opposite  side,  forms  the  base  of  the  nose. 

The  Nasal  Crest. — Beginning  at  the  base  of  the  nasal  spine, 
and  extending  backward  along  the  median  border  of  the  bone, 
is  a  sharp,  irregular  ledge  of  bone,  the  nasal  crest.  This  portion 
of  the  process  articulates  with  the  vomer. 

The  incisor  crest  is  a  continuation  of  the  nasal  crest  anteriorly, 
projecting  beyond  the  nasal  spine  in  the  form  of  a  sharp,  spear- 
like point. 

The  iiicisive  foramen,  ox  foramen  of  Stenson,  is  situated  imme- 
diately back  of  the  incisor  crest,  and  leads  downward  and  for- 
ward from  the  nasal  chamber  toward  the  mouth,  entering  that 
cavity  just  back  of  the  central  incisor  tooth.  This  passage  in 
the  single  bone  is  a  simple  groove,  but  in  the  articulated  skull 
it  becomes  the  anterior  palatine  canal,  which,  after  passing  down- 
ward, opens  on  the  nasal  surface  of  the  palatal  process  by  four 
foramina — the  incisive  foramina,  and  the  foramina  of  Scarpa,  or 
the  naso-palatine  foramina.  These  foramina  transmit  the  naso- 
palatine nerves. 

The  palatal  process  of  the  superior  maxilla  is  subject  to  a 
greater  variety  in  form  than  any  other  portion  of  the  bone, 
this  variation  in  the  articulated  skull  being  the  cause  of  the 
many  different  curves  assumed  by  the  roof  or  dome  of  the 
mouth. 


The  Alveolar  Process. — This  process  forms  the  lower 
margin  of  the  bone,  and  extends  from  the  base  of  the  tuberosity 
behind  to  the  median  Hne  in  front,  at  which  point  it  articulates 
with  the  same  process  of  the  opposite  bone.  It  has  an  outer 
and  an  inner  margin  corresponding  to  the  buccal  and  palatal 
surfaces  of  the  roots  of  the  teeth,  which  are  firmly  imbedded  in 
it.  Its  general  form  from  before  backward  is  that  of  a  gradual 
curve,  somewhat  variable  in  different  bones,  the  extent  of  this 
variation  depending  on  the  type  or  race  to  which  the  bone 
belongs.  The  body  of  the  process  is  made  up  of  an  outer  and  an 
inner  plate,  which  are  connected  by  numerous  septa  of  cancel- 
lated bone.  The  outer  plate  of  the  process  is  continuous  with 
the  facial  and  zygomatic  surface  of  the  body  of  the  bone,  and 
assists  in  forming  these  surfaces.  It  is  quite  thin  and  frail,  and 
the  position  of  the  alveoli  beneath  are  well  shown  by  the 
numerous  vertical  ridges  upon  it.  The  inferior  margin  of  the 
outer  plate  is  reinforced  by  an  additional  thickness  of  bone, 
forming  the  border  of  the  alveolar  sockets.  That  portion  of 
the  plate  supporting  the  molar  teeth  is  heavier  in  general,  and 
the  position  of  the  sockets  are  not  so  plainly  outlined.  The 
inner  plate  of  the  alveolar  process  is  much  heavier  and  stronger 
than  the  outer  plate,  and  extends  from  the  margins  of  the 
alveoli  below  to  the  palatal  process  above.  The  inferior  margin 
of  this  plate  is  not  reinforced  except  in  the  region  of  the 
molars.  The  construction  of  the  inner  plate  is,  to  a  great 
degree,  controlled  by  the  shape  and  position  of  the  palatal  pro- 
cess. In  the  lymphatic  temperament  this  process,  when  articu- 
lated with  its  fellow,  forms  a  flat  or  shallow  dome  to  the  oral 
cavity,  and  in  so  doing  gradually  curves  into  the  alveolar  pro- 
cess, giving  it  additional  thickness.  The  depth  of  the  process 
in  this  type  is  not  great,  and  the  roots  of  the  teeth  are  short  and 
heavy  in  proportion.  In  the  bilious  temperament  the  inner  alveo- 
lar plate  is  deep  and  abrupt,  extending  from  the  inferior  margin 
upward  in  almost  a  perpendicular  direction  to  the  palatal  process, 
which  joins  it  almost  at  right  angles.  The  alveolar  process  gives 
origin  to  one  of  the  cheek  muscles, — the  buccinator, — which  is  at- 
tached to  the  outer  plate  near  its  upper  margin,  and  direcdy  over 
the  space  occupied  by  the  second  bicuspid  and  first  molar  teeth. 


SUPERIOR   MAXILLARY   BONES.  6i 

The  Alveoli  orTooth  Sockets. — These  cavities,which  are  variable 
in  number,  are  formed  by  the  outer  and  inner  plate  of  the 
alveolar  process,  and  by  numerous  connecting  septa  of  bone 
placed  between  the  two  plates.  The  shape  and  depth  of  these 
cavities  is  regulated  by  the  form  and  length  of  the  roots  of  the 
teeth  which  they  support.  The  first  socket,  or  that  next  to  the 
mesial  surface  of  the  bone,  gives  support  to  the  central  incisor 
tooth.  It  forms  almost  a  perfect  cone,  and  has  an  average  depth 
of  half  an  inch.  Its  lower  border  is  circular,  and  the  anterior  or 
labial  portion  describes  a  larger  circle  than  the  posterior  or  palatal 
half.  The  mesial  and  distal  walls  are  somewhat  flattened.  The 
second  cavity,  proceeding  backward  from  the  median  line,  sup- 
ports the  lateral  incisor  tooth.  It  is  also  conic,  but  much 
smaller  than  the  preceding.  It  is  seldom  over  y^  to  y^  of  an 
inch  in  depth.  It  is  much  flattened  on  its  mesial  and  distal  walls, 
giving  the  appearance  of  an  oblong,  rather  than  a  round,  cavity. 
This  socket,  as  well  as  that  for  the  central  incisor,  occupies 
an  almost  vertical  position  in  the  process.  Very  frequently  the 
socket  for  the  lateral  incisor  presents  a  slight  distal  curve  at  its 
upper  extremity.  The  third  socket,  or  that  giving  support  to 
the  cuspid  tooth,  is  much  larger  and  deeper  than  either  of  those 
previously  described.  It  extends  upward,  inward,  and  backward 
to  the  average  depth  of  fg  to  ^  of  an  inch. 

In  transverse  section,  its  labial  wall  presents  a  much  larger 
circle  than  its  palatal  margin.  The  labial  and  distal  walls  are 
much  flattened  and  somewhat  convex.  The  general  direction 
of  this  socket  is  slightly  to  the  distal.  The  socket  which  sup- 
ports the  first  bicuspid  is  usually  divided  from  mesial  to  distal 
by  a  thin  septum  of  bone,  thus  forming  an  outer  or  buccal 
socket,  and  an  inner  or  palatal  socket.  This  division  seldom 
exists  to  the  full  depth  of  the  cavity,  but  usually  begins  about 
midway  of  its  length.  The  lower  margin  of  this  socket  is 
oblong  or  egg-shaped,  its  outer  or  buccal  portion  forming 
a  larger  curve  than  its  palatal.  The  lateral  walls  are  slightly 
concave  or  flattened,  until  the  point  of  separation  is  reached, 
when  they  become  more  circular,  the  alveoli  above  this  point 
becoming  cone-shaped.  It  is  not  uncommon  for  this  socket 
to  be  a  single  cavity,   and  when  thus  formed  it  resembles  a 


flattened  cone,  with  the  buccal  and  palatal  margins  rounded. 
The  next  socket  gives  support  to  the  second  bicuspid  tooth,  in 
most  instances  being  a  single  cavity,  but  in  rare  instances  it  is 
divided  near  its  upper  extremity.  In  general  outline  it  re- 
sembles the  socket  for  the  first  bicuspid. 

The  socket  for  the  first  molar  is  much  larger  than  any  of 
those  previously  described ;  its  inferior  margin  presents  a  cir- 
cular outline  on  its  buccal  and  palatal  portions,  the  former  curve 
being  larger  than  the  latter.  The  mesial  and  distal  walls  are 
flattened  and  slightly  concave.  The  upper  three-fourths  of  this 
socket  is  divided  into  three  separate  compartments,  being  so 
arranged  that  two  are  upon  the  buccal  and  one  upon  the  palatal 
side.  The  septa  separating  the  two  buccal  cavities  from  the 
palatal  cavity  are  heavy  and  strong,  while  that  placed  between 
the  two  buccal  sockets  is  thin  and  frail.  The  two  buccal  cavities 
are  usually  flattened  upon  their  mesial  and  distal  sides.  The 
palatal  socket  is  larger  and  somewhat  deeper  than  the  buccal, 
the  average  depth  of  all  being  about  ^/^  of  an  inch.  The 
socket  for  the  second  molar  is  similar  in  most  respects  to 
that  for  the  first  molar,  except  that  it  is  somewhat  smaller. 
The  same  description  might  answer  for  the  third  molar  socket, 
which  in  general  is  similar  to  the  alveoli  for  the  other  molars. 
It  is  smaller  than  the  second  molar  socket,  and  may  be  a 
single  cavity,  or  it  may  be  divided  into  three  or  more  compart- 
ments. 

Articulations. — The  superior  maxillary  bone  articulates  with 
its  fellow  of  the  opposite  side,  with  the  frontal,  lacrymal,  ethmoid, 
palate,  vomer,  malar,  and  inferior  turbinated  bones.  Occasion- 
ally it  articulates  with  the  sphenoid  bone. 

Attachment  of  Muscles. — The  muscles  attached  to  this 
bone  are  eleven  in  number,  and  are  as  follows  : 

Compressor  nares,  Internal  pterygoid, 

Orbicularis  oris,  Orbicularis  palpebrarum, 

Levator  labii  superioris  alaeque  Levator  labii  superioris  proprius, 

nasi,  Inferior  oblique, 

Levator  anguli  oris,  Buccinator, 

Depressor  alje  nasi,  Masseter. 


SUPERIOR    MAXILLARY   BONES. 


63 


Blood-supply. — The  maxilla  receives  its  vascular  supply 
from  numerous  large  arteries.  They  are  derived  from  the 
alveolar,  infra-orbital,  nasopalatal,  descending  palatal,  eth- 
moidal, nasal,  frontal,  and  facial  branches. 

Development. — The  superior  maxilla  arises  from  four  points 
of  ossification,  which  are  .deposited  in  membrane.  These  four 
centers  make  their  appearance  as  early  as  the  eighth  fetal  week, 
this  early  beginning  making  it  somewhat  difficult  to  accurately 
follow  its  growth.     The  four  centers  are  named,  as  located,  pre- 


FiG.   13.— Left  Superior  Maxillary,  about  the  Third  Year,  Enlarged. 


maxillary,  maxillary,  malar,  and  prepalatal.  The  premaxillary 
nucleus  gives  rise  to  the  incisive  portion  of  the  bone,  or  that 
part  supporting  the  incisor  teeth.  During  early  life  this  division 
of  the  bone  is  separated  from  the  body  of  the  bone,  and  is 
known  as  the  premaxillary  portion  (Fig.  13).  Union  between  the 
premaxillary  portion  and  the  maxilla  proper  takes  place  about 
birth,  and  the  suture  thus  formed  is  visible  on  the  facial  surface 
until  the  sixth  or  seventh  year,  and  on  the  palatine  surface  until 


64  ANATOMY. 

the  adult  period.  The  palatal  suture  extends  as  far  back  as 
the  posterior  border  of  the  anterior  palatal  canal.  This  nucleus 
also  sends  a  narrow  process  upward  which  forms  part  of  the 
outer  boundary  of  the  anterior  narial  aperture.  On  the  palatal 
aspect  it  furnishes  a  speculum  which  surrounds  the  anterior  and 
mesial  walls  of  Stenson's  canal.  The  posterior  limit  of  the 
premaxillary  portion  is  indicated  by  the  suture  on  the  palatal 
surface.  The  maxillary  nucleus  forms  the  greater  portion  of 
the  body  of  the  true  maxilla  and  the  nasal  process.  The  malar 
center  gives  origin  to  the  malar  process,  and  all  that  portion 


Fig.  14. — Developing  Maxillary  Bones  about  the  Fifth  Month  after  Birth. 


external  to  the  infra-orbital  groove.  The  prepalatine  center 
gives  rise  to  the  nasal  surface  of  the  bone  and  that  portion  of 
the  palatal  process  posterior  to  Stenson's  canal. 

Developine7it  of  the  Alveolai-  Process. — This  process  is  rep- 
resented at  birth  by  the  walls  of  a  deep  groove,  in  which  are 
lodged  the  partly  calcified,  deciduous  teeth  and  the  germs  of 
most  of  the  permanent  teeth  (see  Development  of  the  Teeth). 

The  growth  of  the  process  continues  with  the  growth  of  the 
teeth  until,  finally,  at  about  the  seventh  month  after  birth,  the 
dental  organs  are  completely  encased  within  its  walls.     With  the 


SUPERIOR    MAXILLARY   BONES.  65 

decalcification  of  the  roots  of  the  deciduous  teeth  comes  the  loss 
of  the  process  surrounding  them,  and,  as  the  permanent  teeth 
advance  to  take  their  place  in  the  arch,  the  process  is  again  built 
up  about  their  roots. 

The  Maxillary  Sinus,  or  Antrum  of  Highmore*  (Fig. 
12). — This  is  a  large  cavity  situated  within  the  body  of  the 
maxilla.  Its  general  shape  is  that  of  a  pyramid,  with  its  base 
directed  toward  the  median  line,  or  nasal  surface,  its  apex 
pointing  toward  and  extending  into  the  malar  process,  and,  in 
some  instances,  penetrating  the  malar  bone.  The  size  of  the 
cavity  varies  in  different  subjects  and  in  the  opposite  bone  of 
the  same  subject.  The  average  capacity  is  about  three  fluid- 
rams,  but  this  may  be  increased  to  six  or  eight  fluidrams. 
The  size  of  the  bone  and  the  prominence  of  the  malar  process 
control,  in  a  measure,  the  size  of  the  cavity  ;  but  not  infrequently 
the  largest  bone  will  present  the  smallest  sinus.  Sex  also 
appears  to  exert  a  controlling  influence  over  the  capacity  of  the 
cavity,  it  being  greater  in  the  male  than  in  the  female.  In  youth 
the  cavity  is  quite  small,  the  walls  being  much  thicker  propor- 
tionately than  in  the  adult.  The  walls  of  the  sinus  in  the 
matured  subject  are  quite  thin  and  frail,  and  are  four  in  number. 
The  superior  wall  is  formed  by  a  thin  plate  of  bone,  the  floor  of 
the  orbit.  This  surface  is  almost  flat,  and  serves  as  a  roof  to 
the  cavity.  Near  the  anterior  margin  of  this  surface  is  a  thick 
rib  of  bone  which  marks  the  course,  and  forms  one  of  the  walls 
of  the  infra-orbital  canal. 

The  inner  wall,  or  that  looking  toward  the  nasal  surface,  is 
formed  by  the  thin  bony  layer  separating  this  cavity  from  that 
of  the  nares.  The  outer  or  lateral  surface,  formed  by  the  facial 
and  zygomatic  surfaces  of  the  bone,  is  smooth,  and  convex  from 
before  backward.  Near  the  center  of  this  surface  the  cavity 
may  penetrate  the  malar  process,  and  in  the  disarticulated  skull 
would  present  an  opening  at  this  point.  The  inferior  wall  is 
formed  by  the  alveolar  process,  and  is  marked  by  a  number  of 
irregular  eminences  corresponding  to  the  roots  of  the  neighbor- 
ing  teeth.     The  teeth  referred  to  are   generally  the  first  and 

*  Described  separately,  in  preference  to  including  in  general  description  of  the  bone. 
S    . 


second  molars,  and  occasionally  the  second  bicuspid.  It  is  not 
unusual  for  the  roots  of  one  or  more  of  these  teeth  to  penetrate 
the  floor  of  the  sinus,  in  consequence  of  which  the  lining  mem- 
brane of  the  cavity  may  suffer  disease  generated  in  the  teeth. 

The  inferior  wall  is  much  the  strongest  of  the  four,  and,  besides 
the  unevenness  of  the  surface  produced  by  the  tooth-roots,  it 
frequently  supports  a  number  of  thin,  bony  partitions,  which 
may  completely  or  pardy  divide  the  floor  of  the  cavity  into 
numerous  small  compartments. 

The  posterior  portion  of  the  lateral  wall  is  marked  by  the 
posterior  dental  canals,  which  give  passage  to  the  posterior 
nerves  and  blood-vessels.  In  like  manner  the  anterior  portion 
of  the  lateral  wall  is  grooved  for  the  reception  of  the  anterior 
dental  nerves  and  blood-vessels.  Upon  the  inner  wall,  or  that 
forming  the  base  of  the  pyramid,  is  an  opening  which  communi- 
cates with  the  middle  meatus  of  the  nose.  In  the  articulated 
skull  this  opening  is  quite  small,  being  from  y^  to  j^  of  an  inch 
in  diameter.  The  correct  idea  of  this  opening  can  not  be 
obtained  by  studying  the  individual  bone,  as  the  numerous  per- 
forations then  to  be  observed  are  closed  or  partly  closed  by 
articulation  with  adjacent  bones.  The  mucous  membrane  lining 
the  nasal  cavity  enters  the  sinus  through  the  small  aperture  above 
referred  to,  and  forms  a  continuous  lining  over  its  entire  sur- 
face. Cryer  has  thrown  much  light  upon  the  relations  of  the 
maxillary  sinus  to  the  mouth  and  teeth,  and  he  has  demonstrated 
beyond  a  doubt  that  the  relationship  existing  between  the  parts 
is  susceptible  to  extensive  variation.  He  has  shown  that  in 
some  instances  the  cavity  upon  one  side  will  be  large,  with  its 
floor  broken  by  the  tooth-roots,  while  that  upon  the  opposite 
side  will  be  extremely  small  and  far  removed  from  the  root 
apices.  In  fact,  these  researches  have  so  revolutionized  the 
subject  under  consideration  that  the  foregoing  descripdon  is 
only  reliable  in  so  far  as  it  treats  of  the  conditions  most  fre- 
quently met  with. 


THE   PALATE   BONE.  67 

THE   PALATE   BONE. 

T\\&  palate  bones  (Fig.  15),  two  in  number,  are  situated  imme- 
diately posterior  to  the  two  maxillae,  and  with  them  complete 
the  hard  palate.  They  also  assist  in  forming  the  boundaries 
of  the  orbital  and  nasal  cavities,  the  sphenomaxillary,  the 
sphenopalatine,  and  the  pterygoid  fossa,  the  sphenomaxillary 
fissure,  the  posterior  ethmoidal  cells,  and  the  maxillary  sinus. 
When  in  position  in  the  skull,  these  bones  are  wedged  between 
the  maxillae  and  the  sphenoid  bone.  They  are  rectangular  in 
outline,  and  each  bone  presents  for  examination  a  horizontal  and 


PTERVGOLD  FOSSA 


Fig.  15. — The  Two  Palate  Bones  in  their  Natural  Position, 
Dorsal  View. — (Testut.) 


a  vertical  plate,  a  tuberosity,  and  two  processes,  the  orbital  and 
the  sphenoid. 

The  horizontal  plate,  smaller  than  the  vertical,  assists  in 
forming  the  hard  palate,  and  corresponds  to  the  palatal  process 
of  the  maxilla.  In  entering  into  the  construction  of  the  hard 
palate  the  form  of  this  plate  varies  to  the  same  degree  as  the 
palatal  plate  of  the  maxilla.  In  general,  it  is  described  as  quad- 
rilateral in  shape,  having  two  surfaces  and  four  borders.  The 
superior  surface,  which  is  concave  from  side  to  side,  forms  the 
posterior  floor  of  the  nasal  chamber.  The  inferior  surface  com- 
pletes the  hard  palate  posteriorly,  and  presents,  near  its  posterior 
border,  a  transverse  ridge  for  the  attachment  of  one  of  the  mus- 


cles  of  the  soft  palate  (the  tensor  palati) ;  the  anterior  border  is 
serrated  for  articulation  with  the  palatal  process  of  the  maxilla. 
The  posterior  border  is  free,  curved,  and  sharp,  and  marks  the 
posterior  boundary  of  the  hard  palate.  At  the  median  line  this 
border  terminates  in  a  sharp  point,  which,  when  articulated  with 
the  corresponding  bone  of  the  opposite  side,  forms  the  posterior 
nasal  spine  ;  to  this  point  the  azygos  uvulae  muscle  is  attached. 
The  external  border  is  situated  just  below  the  junction  of  the 
horizontal  and  vertical  plates.  In  this  portion  is  a  groove  which 
assists  in  forming  a  portion  of  the  posterior  palatal  canal. 
The  internal  border  is  broad  and  serrated  for  articulation  with 
its  fellow  of  the  opposite  side.  When  the  palate  bones  are  in 
position  in  the  skull,  these  borders  form  a  ridge,  continuing  the 
crest  formed  by  the  palatal  process  of  the  maxilla,  this  crest 
receiving  the  inferior  border  of  the  vomer. 

The  vertical  plate  is  thin  and  frail  and  extends  from  the 
floor  of  the  nasal  chamber  below  to  the  upper  extremity  of  the 
sphenopalatine  notch  above.  It  has  two  surfaces  and  four 
borders. 

The  external  stirface  is  roughened  for  articulation  with  the 
maxilla,  excepting  a  small  triangular  surface  near  the  upper 
extremity,  which  forms  a  portion  of  the  sphenomaxillary  fossa, 
and  a  small  portion  near  the  middle  of  the  surface  close  to  the 
anterior  border,  which  forms  a  portion  of  the  wall  of  the  maxil- 
lary sinus.  Near  the  posterior  boundary  of  this  surface  is  a 
vertical  groove,  which  forms,  when  articulated  with  the  maxilla, 
the  posterior  palatal  canal,  transmitting  the  descending  palatal 
nerves  and  vessels. 

Tlie  internal  stir/ace  is  divided  into  three  shallow  depressions 
by  two  transverse  ridges — the  superior  and  inferior  turbinated 
crests.  The  lower  depression  thus  formed  assists  in  the  con- 
struction of  a  portion  of  the  interior  meatus  of  the  nose.  The 
crest  immediately  above  this  depression  articulates  with  the  infe- 
rior turbinated  bone.  The  central  depression,  the  largest  of  the 
three,  forms  a  portion  of  the  middle  meatus  of  the  nose,  the 
crest  above  articulating  with  the  middle  turbinated  bone.  The 
superior  depression — much  smaller  but  deeper  than  either  of 
those  previously  described — forms  a  large  part  of  the  superior 


THE    PALATE    BONE.  69 

meatus.  The  anterior  border  of  the  vertical  plate  is  thin  and 
sharp,  the  inferior  turbinated  crest  protruding  near  the  center 
of  the  border,  and  forming  the  maxillary  process.  This  process 
assists  in  closing  the  ma.xillary  sinus  by  being  received  into  the 
maxillary  fissure  of  the  maxilla.  At  the  upper  extremity  of  this 
border  is  the  orbital  process,  which  presents  for  examination 
five  surfaces,  three  of  which  are  articular.  The  anterior  or 
maxillary  surface  is  directed  outward,  upward,  and  downward. 
It  is  oblong  in  form  and  articulates  with  the  posterior  superior 
angle  of  the  inner  surface  of  the  maxilla.  The  posterior  or 
sphenoidal  surface  is  directed  backward,  upward,  and  inward, 
and  articulates  with  the  vertical  plate  of  the  ethmoid  bone.  The 
superior  or  orbital  surface  is  triangular  in  form,  extending 
upward  and  outward,  forming  the  posterior  angle  of  the  floor 
of  the  orbit.  The  external  or  zygomatic  surface  is  smooth, 
oblong,  and  directed  outward,  backward,  and  downward,  forming 
a  portion  of  the  sphenomaxillary  fossa. 

The  posterior  border  of  the  vertical  plate  is  irregular  and 
serrated,  and  comes  into  relation  with  the  internal  pterygoid 
process,  terminating  below  in  a  prominent  tuberosity.  This 
presents  three  grooves  or  flutes.  The  inner  receives  the  in- 
ternal pterygoid,  the  outer  the  external  pterygoid  process,  while 
the  middle  groove  completes  the  pterygoid  fossa,  and  gives 
attachment  to  a  portion  of  the  internal  pterygoid  muscle.  This 
process  also  gives  rise  to  the  superior  constrictor  of  the  pha- 
rynx. Passing  through  the  tuberosity  are  a  number  of  small 
canals,  those  on  the  nasal  side  being  the  accessory  palatal 
canals.  Near  the  junction  of  the  tuberosity  with  the  horizontal 
plate  is  the  opening  of  the  posterior  palatal  canal,  and  beyond 
this  the  small  external  palatal  canals. 

The  sphenoidal  process  is  at  the  superior  end  of  the  posterior 
border.  It  is  variable  in  shape  and  curves  upward,  backward, 
and  inward.  It  presents  a  superior,  an  external  and  an  internal 
surface,  and  two  borders — an  anterior  and  a  posterior. 

The  superior  surface,  the  smallest  of  the  three,  is  marked  by 
a  groove,  which  assists  in  forming  the  sphenopalatine  canal. 
This  surface  articulates  with  the  horizontal  portion  of  the 
sphenoidal    turbinated    bone.     The  external   surface  assisis  in 


forming  the  sphenomaxillary  fossa  by  its  anterior  portion,  while 
the  posterior  portion  is  rough  for  articulation  with  the  pterygoid 
plate  of  the  ethmoid  bone.  The  internal  surface  is  instrumental 
in  forming  a  portion  of  the  outer  wall  of  the  posterior  nares, 
and  for  this  purpose  is  smooth  and  concave.  The  anterior 
border  forms  the  posterior  margin  of  the  sphenopalatine  notch. 
The  posterior  border  is  serrated,  and  articulates  with  the  inner 
surface  of  the  pterygoid  process. 

The  superior  border  of  the  vertical  plate  is  divided  by  a  deep 
notch  or  foramen,  which  divides  the  orbital  from  the  sphenoidal 
process.  This  opening  is  the  sphenopalatine  notch  or  foramen, 
and  transmits  the  sphenopalatine  vessels  and  nerves  from  the 
sphenopalatine  fossa  to  the  nasal  chamber. 

The  inferior  border  of  the  vertical  plate  joins  the  external 
border  of  the  horizontal  plate.  Extending  downward  and  back- 
ward from  the  inferior  and  posterior  borders  is  the  pyramidal 
process,  the  borders  of  which  are  serrated  for  articulation  with 
both  pterygoid  plates  of  the  sphenoid  bone. 

Articulations. — The  palate  bone  articulates  with  the  sphe- 
noid, superior  maxilla,  sphenoidal  turbinated,  inferior  turbinated, 
ethmoid,  and  with  its  fellow  of  the  opposite  side. 

Attachment  of  Muscles. — The  following  muscles  are  at- 
tached to  the  palate  bone  : 

Tensor  palati,  Internal  pterygoid, 

Azygos  uvulae,  Superior  constrictor  of  pharynx. 

Blood-supply. — The  arteries  which  supply  this  bone  are 
derived  from  branches  of  the  descending  palatine,  the  spheno- 
palatine, and  pterygopalatine. 

Development. — The  palate  bone  is  developed  from  a  single 
center  deposited  in  membrane.  This  center  makes  its  appear- 
ance about  the  eighth  or  ninth  fetal  week,  near  the  line  of  junc- 
tion between  the  horizontal  and  vertical  plates.  At  birth  these 
plates  are  about  the  same  length,  but  soon  after  this  period, 
w^hen  the  nasal  sinuses  Increase  in  height,  the  vertical  plate 
begins  to  lengthen,  and  continues  to  do  so  until  it  becomes 
nearly  double  the  length  of  the  horizontal  plate. 


INFERIOR    MAXILLARY    BONE.  71 

INFERIOR    MAXILLARY    BONE. 

The  Inferior  Maxillary,  JMandible,  or  Lozver  Jazv  Bone  (Fig. 
16). — This  bone,  having  no  osseous  union  with  the  skull  proper, 
may  be  considered  as  one  of  its  appendicular  elements.  It  is 
the  heaviest  and  strongest  bone  of  the  head,  gives  support  to 
the   sixteen    lower   teeth,   and   serves  as  a  framework    for   the 


Fig.  16. — The  Mandible  or   Inferior  Maxilla.     Right  Side,  External  or  Facial 

Surface. 


lower  half  or  floor  of  the  mouth.  It  is  situated  at  the  lower 
extremity  of  the  face,  and  immediately  below  the  superior 
maxillary  and  malar  bones,  while  its  posterior  extremity  rests 
against  the  glenoid  fossa  of  the  temporal  bone,  forming  a  mov- 
able articulation  with  this  cavity.  In  general,  the  bone  is 
symmetric  in  outline,  and  presents  for  examination  a  horizontal 
portion,  or  body,  and  two  vertical  portions,  or  ravti,  which  in  the 


adult  are  almost  perpendicular  to,  or  at  right  angles  with,  the 
body  of  the  bone. 

The  body,  or  horizontal  portion,  consists  of  two  identical 
halves,  which  meet  at  the  median  line  and  form  a  slight  vertical 
ridge,  the  symphysis.  This  line  indicates  the  point  of  union 
between  the  two  lateral  halves,  which  at  birth  are  usually  sepa- 
rated, but  soon  after  this  period  become  firmly  united.  Each 
lateral  half  of  the  body  presents  two  surfaces — an  external  and 
an  internal ;  and  two  borders — a  superior  and  an  inferior. 

The  external  or  facial  surface  (Fig.  i6)  is  smooth  and 
convex,  and  furnishes  a  number  of  points  for  examination. 
Beginning  at  the  median  line,  the  symphysis  ends  inferiorly  in  a 
prominent  triangular  surface — the  mental  protuberance,  or  chin. 

The  Incisive  Fossa. — Passing  backward  from  the  symphysis, 
and  immediately  above  the  triangular  ridge  which  forms  the 
mental  process,  is  a  decided  but  shallow  depression — the  incisive 
fossa.  This  fossa  gives  origin  to  one  of  the  elevator  muscles  of 
the  chin — the  levator  menti.  Slightly  posterior  to  and  below 
this  fossa,  on  a  line  corresponding  to  the  position  of  the  cuspid 
tooth,  is  an  oblong  depression  for  the  origin  of  the  depressor 
muscle  of  the  lower  lip — depressor  labii  inferioris. 

The  External  Oblique  Line. — Extending  obliquely  across  the 
facial  surface  from  the  mental  process  to  the  base  of  the  vertical 
portion  of  the  bone,  and  continuous  with  its  anterior  margin,  is  a 
well-defined  ridge — the  external  oblique  line.  Near  the  center 
of  this  ridge,  or  below  the  position  occupied  by  the  bicuspid  and 
first  molar  teeth,  is  the  point  of  attachment  of  the  depressor 
muscle  of  the  angle  of  the  mouth — the  depressor  anguli  oris. 
Somewhat  anterior  to  and  above  this  point  is  the  origin  of  the 
depressor  muscle  of  the  lower  lip — the  depressor  labii  inferioris. 
Between  the  line  of  origin  of  the  depressor  anguli  oris  and  the 
inferior  border  of  the  bone  is  a  roughened  surface  for  the  attach- 
ment of  the  platysma  myoides  muscle.  This  roughened  surface 
divides  the  body  of  the  bone  into  an  upper  and  a  lower  portion. 
That  portion  above  is  known  as  the  alveolar  or  mucous  portion, 
while  that  below  is  called  the  basilar  or  non-mucous  portion. 
The  attachment  of  the  platysma  myoides  muscle  at  this  point 
marks  the  lower  boundary  or  floor  of  the  mouth.     The  superior 


INFERIOR    MAXILLARY    BONE. 


or  alveolar  portion  of  the  bone  is  within  the  cavity  of  the  mouth, 
and  is  covered  with  mucous  membrane  and  mucoperiosteum  ; 
while  the  inferior  or  basilar  portion  is  outside  and  below  the 
cavity,  and  is  covered  with  periosteum  similar  to  other  bones. 

The  Mental,  or  Anterior  De7ital  Formnen. — Midway  between 
the  superior  and  inferior  border  of  the  body,  and  usually  below 
the  second  bicuspid  tooth,  is  a  large  foramen, — the  mental  or 
anterior  dental  foramen, — giving  passage  to  the  mental  branches 
of  the  inferior  dental  nerve  and  accompanying  blood-vessels. 
The  position  of  this  foramen  is  not  constant,  but,  as  previously 
stated,  it  is  usually  below  the  second  bicuspid,  or  between  this 
point  and  the  first  bicuspid.  The  buccinator  muscle,  which 
forms  a  large  portion  of  the  lateral  wall  of  the  mouth,  has  its 
origin  from  the  facial  surface  of  the  mandible,  being  attached  to 
the  alveolar  portion  immediately  below  the  molar  teeth. 

The  Internal  Surface  of  the  Body  of  the  Bone  (Fig.  17). 
— The  median  line  is  marked  by  a  slight  vertical  depression, 
representing  the  line  of  union,  and  corresponding  to  the  sym- 
physis externally. 

The  Mylohyoid,  or  Internal  Oblique  Ridge. — The  internal 
surface  is  divided  into  two  portions  by  a  well-defined  ridge — the 
mylohyoid,  or  internal  oblique  ridge.  It  occupies  a  position  closely 
corresponding  to  the  external  oblique  ridge  on  the  facial  sur- 
face. Beginning  near  the  base  of  the  bone  at  the  median  line,  it 
passes  backward  and  upward,  increasing  in  prominence  until  the 
base  of  the  vertical  portion  of  the  bone  is  reached,  into  which  it 
gradually  disappears.  This  ridge  gives  origin  to  the  mylohyoid 
muscle,  which  forms  the  central  portion  of  the  floor  of  the  mouth. 
In  correspondence  to  the  facial  surface  of  the  bone,  the  attach- 
ment of  the  mylohyoideus  muscle  forms  the  dividing  line  between 
the  mucous  membrane  and  mucoperiosteum  covering  the  upper 
portion  of  the  body  of  the  bone,  and  the  periosteum  covering 
the  inferior  portion. 

TJie  Gejiial  Tubercles. — Near  the  lower  third,  at  the  median 
line,  is  a  roughened  eminence — the  genial  tubercles.  Taken 
collectively,  these  are  in  two  pairs — a  superior  and  an  inferior. 
The  superior  pair  (usually  the  largest)  give  origin  to  the  genio- 
hyoglossus  muscle,  and  the  lower  pair  to  the  geniohyoid  muscle. 


74  ,  ANATOMY. 

The  Subling2ial  Fossa. — By  the  side  of  the  genial  tubercles, 
and  above  the  mylohyoid  ridge,  is  a  shallow,  smooth  depression 
— the  sublingual  fossa.  One  of  the  salivary  glands — the  sub- 
lingual— is  partially  supported  in  this  fossa. 

The  Digastric  Fossa. — Below  the  mylohyoid  ridge,  and  near 
the  median  line,  is  a  slight  depression, — the  digastric  fossa, — 
which  affords  attachment  for  the  dieastric  muscle. 


Coronoid        Sigmoid 


Internal  Oblique  Line 

Fig.   17. — The   M.^ndibi.e  or   Inferior   Maxilla.     Right  Side,  Internal  Surface. 

The  Siibmaxillary  Fossa. — In  the  center  of  the  internal  sur- 
face, extending  from  before  backward,  between  the  mylohyoid 
ridge  and  the  lower  border  of  the  bone,  is  an  oblong  depression 
— the  submaxillary  fossa.  In  this  fossa  rests  another  of  the 
salivary  glands — the  maxillary. 

The  Superior  or  Alveolar  Border. — This  border  extends 
from  the  junction  of  the  body,  with  the  vertical  plate  on  one  side, 
to  the  corresponding  point  on  the  other.  The  construction  of 
this  border  is  similar  to  the  alveolar  border  of  the  superior 


INFERIOR    MAXILLARY    BONE.  75 

maxilla.  At  the  anterior  portion  it  is  narrow,  but  gradually 
increases  in  width  as  it  proceeds  backward — in  some  instances 
following  the  line  of  the  body  of  the  bone  ;  in  others,  inclining 
inward,  or  to  the  lingual.  Each  lateral  half  is  marked  by  eight 
sockets,  for  the  accommodation  of  the  sixteen  lower  teeth. 
They  are  smaller  in  proportion  than  the  alveolar  sockets  in  the 
superior  maxilla.  The  socket  nearest  the  median  line  receives 
the  central  incisor  tooth,  and  is  the  smallest  of  the  number.  It 
has  an  average  depth  of  Yn  of  an  inch,  is  conic  from  above 
downward,  oblong  in  transverse  section,  with  its  lateral  walls 
flattened.  The  second  socket  gives  support  to  the  lateral 
incisor  tooth ;  it  is  a  trifle  larger  than  the  central  incisor 
socket,  but  in  other  respects  is  quite  similar.  The  socket  for  the 
cuspid  tooth  is  situated  at  the  anterior  angle  of  this  border,  and 
is  much  larger  and  deeper  than  either  of  the  incisor  sockets.  It 
has  an  average  depth  of  fV  of  ^^  '"ch ;  its  lateral  walls  are 
compressed,  and  sometimes  slightly  concave.  In  transverse 
section  the  labial  wall  forms  a  larger  curve  than  the  internal  or 
lingual  wall.  Passing  backward,  the  next  two  sockets  are  for 
the  support  of  the  bicuspids  ;  they  are  circular  in  outline,  with 
an  average  depth  of  }4  of  an  inch.  The  cavity  for  the  first 
bicuspid  is  usually  a  little  larger  than  that  for  the  second.  In 
rare  instances  one  or  the  other  of  these  sockets  will  be  divided 
for  the  accommodation  of  two  roots.  The  sockets  for  the  first 
and  second  molars  present  a  circular  outline  upon  their  free 
margins,  but  below  they  divide  into  two  flattened,  cone-shaped 
cavities — one  anterior  and  one  posterior.  The  flattened  sides 
of  these  cavities  are  concave  in  the  center,  and  at  their  lower 
third  curve  backward.  The  average  depth  of  these  sockets  is 
j4  of  an  inch.  The  socket  for  the  third  molar,  like  its  superior 
fellow,  is  variable  both  in  form  and  position,  frequently  being 
crowded  inside  or  outside  of  the  tooth-line.  In  some  instances 
it  is  divided  into  two  or  more  compartments.  The  average 
depth  is  not  over  3s  of  an  inch. 

The  alveolar  process,  which  composes  the  superior  border  of 
the  body  of  the  mandible,  differs  from  the  same  process  in  the 
superior  maxilla  in  one  very  important  particular:  instead  of 
the  outer  plate  being  thin  and  frail,  it  is  equally  as  heavy  as  the 


76  ANATOMY. 

inner  or  lingual  plate.  When  the  tooth-line  is  inclined  inward 
from  the  body  of  the  bone,  the  posterior  outer  wall  is  much 
heavier  than  the  interior. 

The  Infejdor  Border  of  the  Body  of  the  Bone. — This  border 
extends  from  a  slight  depression,  to  be  observed  at  the  point  of 
union  between  the  body  and  ramus,  to  the  corresponding  point 
upon  the  opposite  side.  It  is  strong,  rounded,  and  compact, 
and  gives  to  the  bone  the  greatest  portion  of  its  strength. 
Near  its  junction  with  the  ramus  is  the  facial  notch,  so  named 
from  the  facial  artery  passing  over  this  point. 

The  Ramus,  or  Vertical  Portion  of  the  Bone. — This 
vertical  plate  is  quadrilateral  in  outline,  and  presents  two  sur- 
faces— external  and  internal  ;  four  borders — superior,  inferior, 
anterior,  and  posterior ;  and  two  processes — the  condyloid  and 
the  coronoid. 

The  external  surface  is  flat  and  smooth.  Near  the  center  it 
is  slightly  concave  and  roughened  for  the  attachment  of  one  of 
the  muscles  of  mastication — the  masseter. 

TJie  internal  surface  presents  near  the  center  an  oblong  open- 
ing— the  itferior  dental  or  mandibular  foramen — leading  into 
the  inferior  dental  or  mandibular  canal.  Surrounding  this  fora- 
men, on  its  posterior  internal  margin,  is  the  mandibular  spine,  to 
which  is  attached  the  sphenomandibular  ligament.  Running 
obliquely  downward  from  the  base  of  the  foramen,  and  beneath 
the  spine,  is  a  decided  groove, — the  mylohyoid  groove, — which 
accommodates  the  mylohyoid  nerve,  artery,  and  vein,  which  pass 
forward  to  supply  the  floor  of  the  mouth.  Below  and  behind 
this  groove  the  surface  is  roughened  for  the  attachment  of  an- 
other muscle  of  mastication — the  internal  pterygoid. 

Tlie  hferior  Dental  or  Mandibular  Canal. — Beginning  at  the 
foramen  of  the  same  name,  this  canal  enters  the  body  of  the 
bone,  passes  downward  and  forward  horizontally,  until  it  finds 
an  exit  at  the  mental  foramen.  This  canal  lies  immediately 
below  the  alveolar  sockets,  and  from  it  are  given  off  smaller 
canals  which  open  into  the  tooth-sockets  through  minute  fora- 
mina. Near  the  mental  foramen  the  canal  divides  into  a  num- 
ber of  smaller  ones,  which  pass  forward  through  the  substance 
of  the  bone  to  the  sockets  of  the  cuspid  and  incisor  teeth. 


INFERIOR   MAXILLARY   BONE.  77 

The  Superior  Border  of  the  Ramus. — This  border  is  crescent- 
shaped,  and  is  otherwise  known  as  the  sigmoid  notch.  Arising 
from  its  anterior  portion  is  a  flattened,  cone-shaped  process — the 
coronoid  process.  On  its  posterior  portion  is  a  rounded  or 
oblong  eminence — the  condyloid  process.  The  concave  or  cres- 
cent-shaped margin  of  this  border  is  thin  and  smooth  in  front, 
becoming  wider  and  heavier  as  it  approaches  the  condyle. 

Tlie  Coronoid  Process. — The  anterior  margin  of  this  process, 
being  a  continuation  of  the  external  oblique  line,  is  heavier  at 
the  base  than  at  the  apex.  The  outer  surface  is  smooth,  and 
affords  attachment  to  the  masseter,  and  a  few  fibers  of  the  tem- 
poral muscle.  The  internal  surface  is  marked  by  a  vertical 
ridge,  which  passes  downward,  increasing  in  size,  and  finally 
joining  the  internal  oblique  line  at  a  point  posterior  to  the  third 
molar.  The  surface  anterior  to  this  ridge  is  grooved,  and  gives 
attachment  to  a  part  of  the  temporal  muscle  above,  and  the 
buccinator  muscle  below.  The  surface  posterior  to  this  ridge 
affords  attachment  for  the  greater  part  of  the  temporal  muscle. 
The  posterior  border  of  this  surface  is  thin,  and  forms  the  ante- 
rior margin  of  the  sigmoid  notch. 

The  Condyloid  Process. — This  may  be  described  as  the  ex- 
panded extremity  of  the  posterior  border  of  the  ramus,  and  is 
quite  variable  in  form  (see  Occlusion  of  the  Teeth).  It  is 
divided  into  a  superior  or  articular  portion,  and  an  inferior 
portion,  or  neck. 

The  articular  portion  of  the  condyle  is  more  or  less  oblong, 
and  is  convex  above,  fitting  into  the  glenoid  fossa  of  the  temporal 
bone,  and  forming,  with  the  interarticular  cartilage  which  lies 
between  the  two  surfaces,  the  temporomaxillary  articulation. 

The  neck  is  that  constricted  portion  immediately  below  the 
articular  surface.  It  is  flattened  in  front  and  presents  a  pit, — 
the  pterygoid  fossa, — to  which  a  portion  of  the  pterygoid  muscle 
is  attached.  Immediately  below  the  point  of  junction  between 
the  neck  and  the  articular  surface  externally  is  the  condyloid 
tubercle,  to  which  is  attached  the  external  lateral  ligament. 

The  Inferior  Border  of  the  Ramus. — This  border  is  thick, 
rounded,  and  continuous  with  the  lower  border  of  the  body  of 
the  bone.     At  the  point  of  junction  between  this  and  the  pos- 


78  ANATOMY. 

terior  border  is  the  angle  of  the  jaw.  The  angle  has  a  slight 
outward  inclination,  and  is  roughened  for  the  attachment  of  a 
part  of  the  superficial  portion  of  the  masseter  muscle. 

The  anterior  border  has  been  described  in  connection  with  the 
coronoid  process. 

The  posterior  border  is  smooth  and  rounded  on  its  upper  half, 
the  lower  half  being  roughened  for  the  attachment  of  the  stylo- 
maxillary  ligament. 

Attachment  of  Muscles.  —  The  following  muscles  are 
attached  to  the  mandible: 

Buccinator,  Superior  constrictor  of  pharynx, 

Depressor  labii  inferioris,  Masseter, 

Depressor  anguli  inferioris,  Orbicularis  oris, 

Levator  menti,  Internal  and  external  pterj'goid, 

Geniohyoglossus,  Geniohyoid, 

Platysma  myoides,  Mylohyoid, 

Digastric,  Temporal. 

Development.* — On  account  of  its  early  functional  activity, 
the  mandible  is  among  the  first  bones  to  ossify.  Develop- 
ment takes  place  from  six  centers  for  each  lateral  half,  the 
nuclei  being  deposited  as  early  as  the  sixth  or  eighth  fetal 
week,  and  after  their  establishment  the  developmental  process 
takes  place  very  rapidly.  The  six  centers  of  ossification  are 
principally  named  according  to  their  position.  The  early 
preparation  for  the  development  of  the  bone  is  found  in  the 
appearance  of  what  is  known  as  the  mandibular  plates,  which  are 
thrown  out  from  the  sides  of  the  cranial  base,  and  finally  unite  at 
the  median  line.  Not  long  after  this  period  a  cartilaginous  band 
— Meckel's  cartilage — is  developed  in  the  substance  of  the  man- 
dibular plates,  and  it  is  about  this  cartilaginous  framework  that 
ossification  first  takes  place.  The  various  centers  are  distributed 
along  the  line  of  Meckel's  cartilage,  and  are  named  as  follows  : 
Mental,  dentinary,  coronoid,  condyloid,  angular,  and  splenic. 
The  mental  center  provides  for  the  development  of  that  portion 
of  the  bone  between  the  median  line  and  the  mental  foramen. 
The  dentinary  center  forms  the  lower  border  and  outer  plate, 

*  See  "  Development  of  the  Teeth." 


INFERIOR    MAXILLARY    IJONE.  79 

and  provides  for  die  establishment  of  the  crypts  inclosing  the 
developing  tooth-follicles. 

The  coronoid  and  condyloid  centers  are  both  instrumental  in 
constructing  these  processes,  and  the  angular  center  provides 
for  the  angle  of  the  bone.  The  splenic  center  is  somewhat  later 
in   making  its  appearance,  and   from  it  the  inner  plate  of  the 


Fig.   iS. — Chart  Showing  the  Evolution  and  Degeneracy  of  the  Mandible. 


mandible  is  formed,  the  line  ot  union  between  it  and  the  den- 
tinary  center  being  indicated  by  the  mylohyoid  groove.  While, 
as  above  stated,  most  of  the  centers  of  development  are  along 
or  near  the  line  of  Meckel's  cartilage,  the  condyloid  and  coro- 
noid processes  are  developed  from  other  cartilage.     Soon  after 


So  ANATOMY. 

birth  the  two  lateral  halves  of  the  mandible  begin  to  coalesce  at 
the  median  line,  this  union  taking  place  from  below  upward  ;  and 
by  the  eighth  or  tenth  month  union  is  complete  and  the  indi- 
vidual bone  is  established.  The  inferior  maxilla  is  subject  to  a 
continuous  change  in  form,  not  only  in  regard  to  its  general 
contour,  but  also  accommodating  itself  to  the  movements  and 
growth  of  the  teeth,  the  former  taking  place  at  or  about  the 
angle,,  while  the  latter  occurs  in  the  alveolar  portion  of  the 
bone. 

Figure  i8  represents  the  changes  which  take  place  in  the  angle 
of  the  mandible  from  youth  to  old  age.  It  will  be  observed  that 
the  angle  formed  in  the  adult  bone,  with  the  teeth  in  position,  is 
almost  a  right  angle  ;  and  that  in  youth,  with  the  deciduous  teeth 
in  the  alveoli,  the  angle  is  much  more  obtuse,  which  condition  is 
again  approached  in  old  age. 


THE    HYOID    BONE. 

This  is  a  U-shaped  bone  placed  in  the  upper  part  of  the  neck 
at  the  median  line  near  the  base  of  the  tongue.  It  has  no  bony 
connection  with  other  bones  ;  it  is  classed  as  a  floating  bone. 
It  is  made  up  of  a  body  and  four  processes. 

The  body,  or  central  portion,  is  quadrilateral  in  outline,  some- 
what oblong  from  side  to  side,  with  its  anterior  aspect  convex, 
and  presents  a  longitudinal  ridge  which  divides  it  into  a  supe- 
rior and  an  interior  portion.  It  is  also  usually  divided  at  the 
median  line  by  a  slight  vertical  ridge.  At  the  point  of  junction 
between  the  longitudinal  and  vertical  ridges  a  slight  tubercle  is 
formed.  The  anterior  surface  is  given  up  to  the  attachment  of 
muscles.  The  posterior  surface  of  the  body  of  the  bone  is  con- 
cave and  smooth,  and  is  directed  backward  and  downward.  The 
superior  border  gives  attachment  to  the  thyrohyoid  membrane, 
while  the  inferior  border,  which  is  somewhat  thicker,  gives 
attachment  to  the  sternohyoid  and  thyrohyoid  muscles. 

The  processes  known  as  the  greater  and  lesser  cornua  are 
four  in  number,  one  of  each  kind  on  either  side. 

The  greater  cornua  project  backward  and  upward,  and  their 
lower  borders  and  anterior  surfaces  are  occupied  with  muscles. 


THE    HYOID    BONE.  8l 

The  thyrohyoid  ligament  is  attached  to  the  posterior  terminal 
corner. 

The  lessei^  cornua  are  short  conical  pieces  of  bone,  and  project 
upward  and  backward  from  the  extremities  of  the  body  of  the 
bone.     They  give  attachment  to  the  stylohyoid  ligaments. 

Development. — Ossification  takes  place  from  five  centers, 
one  for  the  body  of  the  bone  and  one  for  each  cornua. 

Attachment  of  Muscles. — 

Geniohyoglossus,  Sternohyoid, 

Geniohyoid,  Lingualis, 

Thyrohyoid,  Omohyoid, 

Mylohyoid,  Digastric, 

Hyoglossus,  Middle  constrictor. 

The  thyrohyoid  ligament,  as  well  as  the  stylohyoid,  and  the 
thyrohyoid  membrane  are  also  attached  to  this  bone. 


CHAPTER  III 

THE  TEMPOROMANDIBULAR  ARTICULATION.  — THE  MUSCLES  OF 
MASTICATION. 

TEMPOROMANDIBULAR    ARTICULATION. 

Although  external  to  the  cavity  of  the  mouth,  this  articula- 
tion is  so  closely  associated  with  the  masticatory  function  that 
it  seems  important  that  a  brief  description  of  its  construction 


Fig.  19. — Temporomandibular  Articulation. 

and  action  should  be  given.  It  receives  its  name  from  the  two 
bones  which  enter  into  its  formation — the  temporal  and  the  man- 
dible, or  inferior  maxillary. 

This  joint  is  the  seat  of  motion  in  the  mandible,  and  entering 
into  its  construction  are  bones,  ligaments,  cartilage,  and  synovial 
membrane,  these  being  the  tissues  essential  to  all  diarthrodial 


% 


TEMPOROMANDIBULAR   ARTICULATION.  83 

or  movable  articulations.  The  various  movable  joints  of  the 
body  are  classified  according  to  the  nature  of  the  movement, 
and  correspond  to  the  mechanical  actions  known  as  hinge  joint, 
ball-and-socket  joint,  gliding  joint,  pulley  joint,  etc.  The  tem- 
poromandibular joint  is  of  the  diarthrodial  class,  and  the  move- 
ments which  it  possesses  are  a  combination  of  the  gliding 
movement  (arthrodia)  and  of  the  hinge  movement  (ginglymus). 
The  osseous  parts  entering  into  the  formation  of  the  joint  are 
the  anterior  portion  of  the  glenoid  fossa  of  the  temporal  bone 
and  the  condyloid  process  of  the  mandible  (Fig.  19). 

The  glenoid  fossa  may  be  described  as  an  oblong  cavity, 
with  its  base  directed  upward,  being  bounded  anteriorly  by  a 
heavy  bony  ridge  (the  anterior  root  of  the  zygoma),  posteriorly 
by  an  irregular,  flattened  portion  of  the  bone  (the  tympanic  plate 
of  the  petrous  portion).  Internally  by  a  union  of  the  anterior 
and  posterior  boundaries,  and  externally  by  the  middle  root  of 
the  zygoma.  The  floor  of  the  fossa  is  traversed  by  a  well- 
marked  fissure — the  glenoid  fissure  (fissure  of  Glaserius) — which 
divides  the  fossa  into  two  portions,  an  anterior  and  a  posterior. 
The  anterior  half  is  deeper  and  more  concave  than  the  posterior, 
and  is  die_  articulating  portion,  being  occupied  by  the  condyle, 
while  the  posterior  half  gives  lodgment  to  the  parotid  gland. 

The  condyloid  process  of  the  mandible  having  been  de- 
scribed with  that  bone,  in  this  connection  reference  will  be  made 
to  the  variety  of  forms  which  it  presents,  and  the  influence 
which  it  exerts  over  the  nature  of  the  tooth  occlusion.  This 
process,  when  narrow  and  oblong  (Fig.  19),  closel)'  resembles 
the  ginglymus,  or  hinge  joint,  and  will  be  accompanied  by  teeth 
presenting  deep,  penetrating  cusps,  forming  a  positive  and  well- 
locked  occlusion  (Fig.  20,  B),  with  little  or  no  lateral  motion.  If 
the  condyle  presents  the  appearance  shown  in  figure  21,  which 
resembles  the  enarthrodia,  or  ball-and-socket  joint  (although  it 
can  not  be  considered  as  such),  the  teeth  associated  with  such  a 
formation  will  be  provided  with  short,  rounding  cusps,  and  the 
occlusion  will  be  loose  and  wandering  (Fig.  20,  A).  This  differ- 
ence in  the  form  of  the  condyle  will  be  accompanied  by  a  corre- 
sponding variation  in  the  concavity  of  the  glenoid  fossa.  Not 
only  does  the  osseous  structure  in  the  joint  partake  of  individual 


characteristics,  but  likewise  the  muscles  and  ligaments  ;  their 
functions  being  to  operate  the  articulation,  they  are  developed 
in  accordance  with  the  action  required  of  them,  which  action  is. 


in   a   measure,  dependent  upon   the  conditions   existing  in  the 
mouth. 

Both  the  condyle  and  the  glenoid  fossa  are  covered  with  ar- 


TEMPOROMANDIBULAR  ARTICULATION.  85 

ticLilar  cartilage.  In  the  latter  this  membrane  extends  over  its 
anterior  border,  to  facilitate  the  play  of  the  joint.  The  condyle 
is  held  in  position  in  the  fossa  by  three  ligaments — the  cap- 
sular, the  sphenomaxillary,  and  stylomaxillary.  The  caps^dar 
liga7nentis  divided  into  four  portions — anterior  and  posterior,  ex- 
ternal and  internal.    The  anterior  portion  consists  of  a  few  fibers 


styloid  Process         Capsular  Lij 


Internal  Lateral  Li; 


Stylohyoid  Ligament 

Stylomaxillary  Ligament 
Fig.  22. — Temporomaxillary  Articulation — Internal  View. — {Deaver.) 


connected  with  the  anterior  margin  of  the  fibrocartilage,  at- 
tached below  to  the  anterior  margin  of  the  condyle  and  above 
to  the  front  of  the  glenoid  ridge.  The  posterior  portion  is 
attached  above  just  in  front  of  the  glenoid  fissure,  and  is  in- 
serted into  the  posterior  margin  of  the  ramus  of  the  maxilla 
just  below  the   neck  of  the   condyle.     The    external    portion. 


otherwise  known  as  the  external  Hgament,  is  the  strongest  por- 
tion of  the  capsular  ligament.  It  has  a  broad  attachment  above 
to  the  zygoma,  from  which  point  it  passes  downward  and  back- 
ward, and  is  inserted  into  the  outer  side  of  the  neck  of  the 
condyle.  The  internal  portion,  or  short  internal  lateral  ligament, 
is  composed  of  well-defined  fibers,  having  a  broad  attachment 


External  Lateral  Ligament 


Styloid  Process 
Stylohyoid  Ligament 
Stylomaxillary  Ligament 

Fig.  23. — Temporomaxillary  Articulation — External  View. — [Deaver.) 

above  to  the  inner  edge  of  the  glenoid  fossa  and  to  the  alar 
spine  of  the  sphenoid  bone ;  below  it  is  inserted  into  the  inner 
side  of  the  neck  of  the  condyle. 

The  sphenomaxillaT-y ^  or  long  internal  lateral  ligament,  is  a 
thin,  loose  band,  situated  some  distance  from  the  joint  proper. 


TEMPOROMANDIBULAR   ARTICULATION.  87 

and,  as  its  name  implies,  has  its  attachment  above  to  the  alar 
spine  of  the  sphenoid  bone,  and  also  to  that  portion  of  the  tem- 
poral bone  contiguous  to  it.  It  passes  downward  and  forward, 
and  is  inserted  into  the  mandibular  spine  of  the  maxilla. 

The  stylomaxillary  ligament  extends,  from  the  styloid  process 
of  the  temporal  bone,  downward  and  forward,  to  be  inserted 
into  the  posterior  border  of  the  ramus  of  the  inferior  maxilla,  at 
a  point  between  the  masseter  and  internal  pterygoid  muscles. 

The  interM'ticidar  Jibrocartilage  is  an  oval  sheet  placed  be- 
tween the  two  articulating-  surfaces.     It  is  thinnest  at  the  center 


Fig.  24. — Showing  Variation  in  the  Shape  of  the  Condyles. 

and  becomes  thicker  as  the  margins  of  the  fossa  are  approached, 
at  which  point  it  is  connected  with  the  fibers  of  the  capsular 
ligament.  Being  placed  immediately  between  the  two  articular 
surfaces  it  divides  the  joint  into  two  separate  synovial  cavities. 
Each  of  these  synovial  cavities  is  occupied  by  a  synovial  mem- 
brane, that  occupying  the  upper  compartment  being  the  largest, 
and  passes  from  the  margins  of  the  glenoid  fossa  above  to  the 
upper  surface  of  the  interarticular  cartilage  below.  The  mem- 
brane which  occupies  the  lower  cavity  is  smaller,  and  passes 
from  the  under  surface  of  the  interarticular  cartilage  above  to 
the  margins  of  the  condyle  below.      The  blood-siipply  to  this 


articulation  is  derived  from  the  temporal,  middle  meningeal,  and 
ascending  pharyngeal  arteries. 

The  nerves  are  derived  from  the  masseteric  and  auriculo- 
temporal. 

The  movements  of  this  articulation  present  as  great  a  range 

Temporal  Muscle 


Superficial  Temporal 

Artery 
Facial  Ne 


Buccinator  Muscl 

Masseteric  Nerve   / 
Masseteric  Artery 


Fig.  25. — Temporal  Muscle. — [Deaver.) 


as  any  other  joint  in  the  human  body.  While  the  chief  move- 
ment is  of  the  ginglymoid  or  hinge  character,  brought  into  play 
in  simple  depression  and  elevation  of  the  mandible,  it  also  has 
the  power  of  extension  and  retraction,  may  be  rotated  from  side 
to   side,  together  with  all   the   motions   intermediate    between 


TEMPOROMANDIBULAR   ARTICULATION.  89 

these.  When  the  mandible  is  depressed,  the  condyle  moves  on 
the  fibrocartilage,  and  at  the  same  time  glides  forward  and 
slightly  downward  until  it  rests  on  the  anterior  border  of  the 
glenoid  fossa  ;^this  movement  does  not  extend  sufficiently  to 
allow  the  condyle  to  rest  upon  the  extreme  summit  of  the 
border,  except  in  cases  of  excessive  movement,  as  in  yawning, 
when  the  condyle  may  glide  over  the  summit  and  the  joint 
become  disarticulated.  When  the  mandible  is  elevated,  the 
condyle  slides  backward  and  upward,  and  at  the  same  time  the 
fibrocartilage,  which  has  extended  with  it,  also  retracts  until  the 
condyle  is  settled  in  the  fossa.  The  movement  of  extension  and 
retraction  is  by  a  horizontal  gliding  action,  by  which  the  mandible 
is  thrust  forward  and  drawn  back  again.  In  this  movement,  as 
well  as  in  the  one  previously  described,  both  condyles  are  sim- 
ilarly and  simultaneously  engaged.  The  lateral  or  triturating 
movement  is  made  in  an  oblique  direction.  This  consists  in  a 
rotation  of  the  condyles  within  the  fossae,  the  cartilage  gliding 
obliquely  forward  and  outward  on  one  side,  and  backward  and 
inward  on  the  other,  this  action  taking  place  alternately.  This 
movement  is  more  or  less  developed  in  accordance  with  the 
nature  of  the  occlusion,  being  favored  by  those  teeth  possessing 
but  little  cusp  formation,  with  a  consequent  loose  and  wandering 
occlusion  ;  while  in  that  type  of  tooth  associated  with  a  long 
overbite  and  deep  penetrating  cusps,  forming  a  firm  and  well- 
locked  occlusion,  this  movement  will  be  but  little  developed.  If 
this  movement  be  employed  to  throw  the  symphysis  to  one  side 
and  back  again,  and  not  from  side  to  side,  the  condyle  of  that 
side  rotates  in  the  glenoid  fossa,  while  the  condyle  of  the  oppo- 
site side  is  drawn  forward  and  inward. 

The  Muscles  of  Mastication. 

Occupying  the  back  part  of  the  side  of  the  face,  and  forming 
an  independent  group,  are  four  muscles,  usually  classed  as  the 
muscles  of  mastication.  While  this  is  true  to  a  great  degree, 
they  are  not  the  only  muscles  brought  into  action  during  this 
process.  They  are  the  masseter,  temporal,  internal  pterygoid, 
and  external  pterygoid.  The  masseter,  temporal,  and  internal 
pterygoid  lift  or  close  the  lower  jaw,  the  principal  function  of 


go  ANATOMY. 

the  external  pterygoid  being  to  extend  the  lower  jaw  so  that  the 
lower  teeth  pass  beyond  the  upper.  The  muscles  which  open 
the  jaws,  such  as  happens  when  the  head  is  thrown  backward, 
are  the  muscles  of  the  neck. 

The  masseter  muscle  is  made  up  of  a  strong  quadrate 
sheet,  consisting  of  two  distinct  layers  extending  from  the 
zygomatic  arch  to  the  mandible.    The  layers  of  which  the  muscle 


Interarticular  flbro-cartilage 


External  pterygoid 


Internal  pterygoid 


Fig.  20. — The  Pterygoid  Muscles. — [Morris.) 


is  composed  differ  somewhat  in  size  as  well  as  in  the  directions 
which  they  take. 

Origin. — The  superficial  layer,  much  the  stronger  and  larger 
of  the  two,  arises  from  the  lower  border  of  the  malar  bone,  and 
from  the  anterior  two-thirds  of  the  zygomatic  arch.  The  deep 
layer  arises  from  the  posterior  third  of  the  lower  border  and 
from  nearly  all  the  internal  surface  of  the  zygomatic  arch. 

Insertion. — After  passing  downward  and  backward  it  is  in- 
serted into  the  outer  surface  of  the  ramus  of  the  mandible. 
The  deep  layer,  passing  downward  and  slightly  forward,  mingles 


TEMPOROMANDIBULAR   ARTICULATION.  91 

with  some  of  the  fibers  of  the  superficial  portion,  and  is  finally 
inserted  into  the  upper  half  of  the  ramus  of  the  mandible. 

Action. — To  draw  slighdy  forward,  and  by  its  superficial  layer 
to  close  the  jaw.  "  In  closing  the  jaw  it  acts  with  less  mechan- 
ical disadvantage  than  is  usual  with  muscles.  When  the  pres- 
sure to  be  overcome  is  exerted  upon  the  back  teeth,  the  arm  of 
the  lever  upon  which  the  power  acts  is  almost  as  long  as  that 
which  intervenes  between  these  teeth  and  the  fulcrum.  This 
fulcrum  is  not  at  the  temporomaxillary  joint,  but  at  a  point  below 
the  neck  of  the  mandible,  corresponding  very  nearly  to  the 
lower  attachment  of  the  internal  lateral  ligament.  Moreover, 
the  resultant  force  of  the  muscle  acting,  as  it  does,  upward  and 
forward,  is  perpendicular  to  the  lever,  which  may  roughly  be 
described  as  a  bar  extending  downward  and  forward  from  the 
neck  of  the  mandible  to  the  point  of  the  chin."      (Morris.) 

Relations. — It  is  covered  superficially  by  the  skin  and  fascia 
of  the  platysma  myoides,  by  the  risorius  and  the  masseter  fascia, 
by  the  parotid  gland  and  ducts,  facial  veins,  and  portions  of  the 
facial  nerve.  Deeply,  the  muscle  lies  in  contact  with  the  ramus 
of  the  jaw  and  the  buccinator  muscle,  being  separated  from  the 
latter  by  a  layer  of  fat.  A  small  portion  of  the  temporal  mus- 
cle also  comes  in  relation  to  the  deeper  lying  portion. 

The  temporal  muscle  is  covered  by  a  strong  membrane, 
the  temporal  fascia,  which  arises  from  the  temporal  ridge,  and 
is  inserted  into  both  the  inner  and  outer  portions  of  the  upper 
border  of  the  zygomatic  arch.  Near  this  point  it  divides  into 
two  distinct  layers,  one  passing  to  the  inner,  the  other  to  the 
outer  margins  of  the  zygoma.  Below  the  fascia  is  continuous 
with  the  masseteric  fascia.  Passing  downward  from  this  to  the 
inferior  borders  of  the  ramus  of  the  jaw,  it  envelopes  the  mas- 
seter muscle.  The  muscle  itself  is  radiating  and  fan-shaped  in 
form,  located  in  the  temporal  fossa,  from  which  point  it  descends 
to  the  coronoid  process  of  the  mandible. 

Origin. — From  nearly  the  entire  surface  of  the  temporal 
fossa,  from  the  temporal  ridge,  from  the  entire  surface  of  the 
temporal  fascia,  down  to  its  lower  attachment  to  the  zygomatic 
process. 

Insertion. — Into  the  coronoid  process  ot  the  mandible. 


Action. — To  close  the  lower  jaw,  some  of  its  fibers  drawing 
the  jaw  backward  after  the  other  muscles  have  protruded  it. 

Relations. — Its  superficial  portion  is  covered  by  the  temporal 
fascia  which  separates  it  from  some  of  the  auricular  muscles  ; 
branches  of  the  facial  nerve,  the  auriculotemporal  nerve,  and  a 
portion  of  the  epicranial  aponeurosis.  The  temporal  fossa 
and  the  external  pterygoid  muscles  are  in  relation  with  it 
deeply. 

The  internal  pterygoid  muscle  is  a  thick,  quadrilateral, 
sheet-like  muscle,  and  receives  its  name  from  its  origin  and  rela- 
tive position. 

Origin. — From  the  inner  surface  of  the  external  pterygoid 
plate;  the  tuberosity  of  the  palate  bone  and  a  small  portion  of 
the  maxilla. 

Insertion. — Into  the  internal  surface  of  the  ramus  of  the  man- 
dible at  its  lower  and  posterior  borders  and  extending  as  high 
as  the  mandibular  foramen  and  mylohyoid. 

Action. — To  close  the  jaw  and  at  the  same  time  draw  it  back- 
ward and  throw  it  toward  the  opposite  side.  "  The  same  re- 
marks which  were  made  with  respect  to  the  very  small  loss  ol 
mechanical  advantage  in  the  masseter  miiscle  apply  to  this 
muscle.  When  closed  it  will  draw  the  jaw  forward  ;  and  also 
it  will  help  the  external  pterygoid  in  drawing  the  ramus  of 
its  own  side  toward  the  middle  line."      (Morris.) 

Relations. — Superficially  the  internal  maxillary  vessels,  the 
external  pterygoid  muscle,  the  internal  lateral  ligament,  the 
inferior  dental  and  hngual  nerves.  Deeply,  the  submaxillary 
glands ;  the  tensor  palati  and  superior  constrictor  of  the 
pharynx,  as  well  as  the  stylohyoid  and  posterior  border  of  the 
digastric  muscles. 

The  external  pterygoid  muscle  is  composed  of  two  trian- 
gular sheets,  one  passing  in  a  horizontal  and  the  other  in  a 
vertical  direction.  It  receives  its  name  from  its  attachment  to 
the  pterygoid  process  of  the  sphenoid  bone  as  well  as  its  rela- 
tion to  its  companion  muscle,  the  internal  pterygoid. 

Origin. — It  is  composed  of  two  distinct  heads,  an  upper  and 
a  lower.  The  upper  head  arises  from  the  greater  wing  of  the 
sphenoid  bone,  from  the  internal  pterygoid  ridge,  and  external 


TEMPOROMANDIBULAR   ARTICULATION.  93 

to  the  foramen  ovale  and  foramen  spinosum.  The  lower  head 
arises  from  the  outer  surface  of  the  external  pterygoid  plate. 

Insertion. — The  upper  head  is  inserted  into  the  inter-articular 
fibrocartilage,  into  the  capsule  of  the  joint  as  well  as  the  neck 
of  the  condyle.  The  lower  head  is  inserted  into  the  neck  of  the 
condyle. 

Action. — To  draw  the  condyle  and  inter-articular  fibrocartilage 
forward  and  inward.  "  The  combination  of  these  two  move- 
ments produces  the  oblique  movement  of  the  lower  molar  teeth 
of  one  side,  forwards  and  inwards  with  respect  to  the  upper 
molars  which  are  their  opponents.  It  should  be  observed  also 
that  this  inward  movement  of  one  side  is  the  agent  by  which  the 
ramus  of  the  opposite  side  is  moved  outwards.  To  assist 
in  opening  the  mouth  by  depression  of  the  lower  jaw.  As  the 
transverse  axis  of  this  movement  passes  through  the  mandible 
at  two  points  situated  below  the  necks  of  the  rami,  it  follows 
that  a  forward  movement  of  the  condyles  and  necks  will  assist  in 
the  backward  movement  of  the  angles  and  body  which  accom- 
panies the  depression  of  the  mandible."      (Morris.) 

Relations. — Superficially,  some  of  the  fibers  of  the  internal 
pterygoid,  the  temporal,  and  part  of  the  masseter  muscle. 
Deeply,  the  internal  pterygoid  muscle,  the  middle  meningeal 
and  inferior  dental  vessels,  internal  maxillary  vessels,  and  mas- 
seteric and  posterior  deep  temporal  nerves  passing  behind  or 
through  the  attachment  to  the  upper  head  ;  the  inferior  dental 
and  lingual  gustatory  nerves  beneath  the  lower  head. 


CHAPTER  IV. 

A  GENERAL  DESCRIPTION  OF  THE  TEETH.-THE  PERMANENT  TEETH  : 
CLASSIFICATION,  SURFACES,  ETC.-THE  ROOTS  OF  THE  TEETH. 
—THE  DENTAL  ARCH. 

A  GENERAL   DESCRIPTION   OF  THE  TEETH. 
A  Tooth  (Fig.  27). — One  of  the  thirty-two  speciahzed  organs 
for  the  seizure  and  mastication  of  food,  placed  at  the  entrance 
to  the  alimentary  canal  (the  mouth).     The   typical   form  of  a 

tooth  is  a  modified  cone  or  combi- 
nation of  cones,  and  is  composed  of 
two  fundamental  parts — the  crown 
and  the  7'oot  or  roots.  The  crown  is 
that  part  which  projects  beyond  the 
gum  and  is  visible  in  the  mouth ; 
while  the  root  is  that  part  which  is 
implanted  in  the  bone  and  covered 
by  the  gum.  Intervening  between 
..^  these  two  extremities,  and  usually 
occupying  a  portion  of  each,  is  a 
third  division — the  neck. 

Completely  covering  the  crown  of 
a  tooth  is  a  hard,  vitreous-like  sub- 
stance, enamel ;  *  the  root  Is  covered 
by  a  hard,  bone-like  substance,  ce^n- 
jTiQ  27.  entum  ;*  while  the  interior  or  body 

of  the  organ  is  composed  of  a  hard 
substance  closely  resembling  bone,  the  dentin.*  The  neck  of 
a  tooth,  which  serves  to  unite  the  crown  to  the  root,  and  which 
is  usually  formed  at  the  expense  of  each,  is  covered  partly  by 
enamel  and  partly  by  cementum.  This  brief  description  in  the 
singular  number  can  best  be  continued  in  the  plural.     Teeth  are 

*  See  Tissues  of  the  Teeth,  part  II. 
94 


A    GENERAL   DESCRIPTION   OF   THE   TEETH.  95 

classified  according  to  their  form,  which  is  always  in  accordance 
with  their  function,  into  swiple  and  complex.  In  the  simple  class 
(Fig.  27)  the  single  modified  cone  is  the  predominating  form,  the 
free  extremity  of  the  crown  serving  as  the  base  of  the  cone,  while 
the  apex  is  formed  by  the  free  end  of  the  root.  Included  in  this 
same  classification  are  those  teeth  which  are  made  up  of  a  double 
cone,  or  a  simple  cone  and  an  inverted  cone  attached  to  each 
other  at  a  common  base.  The  purposes  for  which  such  teeth 
are  adapted  are  those  of  grasping,  incising,  and  tearing,  and 
they  are  usually  so  arranged  that  the  free  extremities  of  their 
crowns  interlock  or  overhang  each  other. 

In  the  complex  class  (Fig.  28)  the  external  form  of  the  tooth 
is  produced  by  a  combination  of  cones,  some  of  which  are  simple, 
others  inverted,  but  all  uniting  at  a  com- 
mon base — the  neck  of  the  tooth.  In  this 
class  the  simple  cones  form  the  roots  of 
the  teeth,  while  the  crowns  are  made  up 
of  a  number  of  smaller  cones,  much  modi- 
fied. These  teeth  are  adapted  to  crush- 
ing and  grinding,  and  are  less  inclined  to 
interlock  during  active  service. 

The  teeth  are  divided  into  two  grand 
divisions — those  of  infancy  and  child- 
hood, called  deciduotis  or  temporary 
teeth,  and  those  of  the  adult  period, 
known  as  permanent  teeth.  The  latter 
class  being  most  important,  will  first  receive  consideration. 

The  Permanent  Teeth  (Fig.  29). — The  permanent  teeth, 
thirty-two  in  number,  are  divided  into  those  of  the  superior 
portion  of  the  mouth,  tipper,  and  those  of  the  inferior  portion, 
lower.  In  number  they  are  equally  divided,  each  jaw  giving 
support  to  sixteen.  They  are  firmly  imbedded  in  the  alveolar 
sockets  of  three  of  the  bones  of  the  mouth,  the  upper  sixteen 
being  attached  to  the  two  superior  maxillary  or  upper  jaw-bones, 
and  the  lower  sixteen  to  the  mandible,  inferior  maxillary,  or 
lower  jaw-bone.  As  above  referred  to,  the  attachment  of  the 
teeth  to  the  bones  is  by  implantation  in  sockets,  the  alveoli  (see 
description,  "Bones  of  the  Mouth").     In  this  attachment  there 


96  ANATOMY. 

is  a  special  development  of  bone,  closely  modeled  to  the  roots  of 
the  teeth,  and  which  is  subservient  to  the  ever-varying  changes 
which  take  place  during  the  development  of  the  organs.  The 
joint  thus  formed  between  the  roots  of  the  teeth  and  the  alveoli 
is  of  the  immovable  or  synarthrodial  class,  and  is  styled  gom- 
pliosis.  Intervening  between  the  roots  of  the  teeth  and  the  walls 
of  the  alveoli  is  a  delicate  membrane — the  alveolodental  mem- 
brane. 

Before  continuing  the  description  of  the  teeth,  a  further  clas- 
sification, which  refers  alike  to  the  upper  and  lower  organs, 


Lingual  Surfaces 


Labial  and  Buccal  Surfac 


I  Surfaces  Labial  ! 

Fig.  29. — The  Permanent  Teeth. 


must  be  presented.  This  classification  is  derived  from  the 
function  and  form  of  the  teeth.  Figure  29  shows  the  thirty-two 
teeth  removed  from  the  jaws  and  placed  side  by  side  in  two 
straight  lines.  In  the  center  is  a  perpendicular  line,  which 
corresponds  to  the  median  line  or  center  of  the  mouth,  the  teeth 
at  either  extremity  being  those  which  occupy  the  back  part  of 
the  mouth.  Without  confining  the  description  to  either  the 
upper  or  lower  teeth,  it  will  be  observed  that  the  first  two 
teeth  upon  either  side  of  the  median  line  are  similarly  formed, 
and  all  four  are  called  incisors  {incidere,  to  cut);  the  two  largest 


A   GENERAL   DESCRIPTION   OF  THE   TEETH.  97 

incisors,  being  nearest  the  median  line  or  center,  are  called 
central  incisors  ;  while  the  two  smallest,  being  placed  at  the  side 
of  the  centrals,  are  known  as  lateral  incisors  {lateralis,  the  side). 
The  third  tooth  from  the  median  line  upon  either  side  is  the 
cuspid  (cuspis,  a  point),  so  named  from  possessing  a  single  cusp 
or  point.  Passing  to  the  right  or  left  on  the  chart,  or  backward 
in  the  mouth,  the  fourth  and  fifth  teeth  from  the  median  line  are 
the  bicuspids  {bi,  two ;  ciispis,  a  point),  having  two  points  or 
cusps.  The  bicuspid  nearest  the  median  line  is  th.&  first  bicuspid ; 
that  most  distant  from  the  median  line  is  the  second  biacspid. 
The  sixth,  seventh,  and  eighth  teeth  from  the  median  line  upon 
either  side  are  those  of  another  class,  the  molars  (mola,  a  mill- 
stone), being  named  according  to  their  function,  that  of  crushing 
or  grinding  the  food.  Proceeding  from  before  backward,  the 
molars  are  denominated  first  molar,  second  molar,  and  tJiird 
molar.  To  sum  up,  the  names  and  number  of  the  permanent 
teeth  may  be  given  by  the  dental  formula,  as  follows : 

Incisors, f  Bicuspids,      I 

Cuspids, 1  Molars,       \ 


The  Surfaces  of  the  Teeth. — The  crown  of  each  tooth  pre- 
sents five  surfaces,  which  are  variously  named,  in  accordance 
with  the  duty  which  they  are  called  upon  to  perform  or  suggest- 
ive of  their  location.  The  outer  surface  of  the  incisors  and 
cuspids,  or  that  contiguous  to  the  lips  (labia),  is  called  the  labial 
surface  ;  the  corresponding  surface  of  the  bicuspids  and  molars, 
or  that  contiguous  to  the  cheeks  (buccae),  is  the  buccal  surface. 
That  surface  of  both  upper  and  lower  teeth  which  faces  the 
palate  or  tongue  is  characterized  as  the  lingual  surface. 

The  proximate  surfaces  of  the  teeth  are  named  with  regard 
to  their  relation  to  the  median  line,  those  surfaces  nearest  to 
this  point  being  called  mesial,  those  most  distant,  distal.  In 
addition  to  these  four  surfaces,  which  represent  what  might  be 
termed  the  sides  of  the  teeth,  a  fifth  surface  is  present,  that 
which  occludes  with  the  teeth  of  the  opposite  jaw,  and  called 
the  occlusal  surface. 

In  the  incisors  and  cuspids  this  surface  is  formed  by  the  con- 
verging of  the  labial  and  lingual  surfaces,  forming  an  edge  to  the 
7 


free  extremity  of  the  crown,  named,  from  its  action  in  mastica- 
tion, the  incisive  or  ciitting-edge.  In  the  bicuspids  and  molars 
the  various  sides  of  the  crowns  remain  nearly  parallel  to  each 
other  throughout  their  extent,  thus  providing  a  surface  nearly 
equal  to,  or  greater  than,  any  of  the  others,  and  one  well  adapted 
to  the  purposes  for  which  it  is  intended — that  of  grinding  or 
crushing  the  food. 

The  Roots  of  the  Teeth. — The  upper  incisors  and  cuspids 
are  each  provided  with  one  root ;  the  upper  first  bicuspid  may 
have  one  or  two  roots,  most  frequently  the  latter ;  while  in  the 
second  bicuspid  a  single  root  is  usually  present.  The  upper 
first  and  second  molars  are  each  supported  in  the  jaw  by  three 
roots,  and  while  in  the  upper  third  molar  three  roots  are  most 
common,  the  number  is  quite  variable,  ranging  from  a  single 
cone-shaped  root  to  three,  four,  five,  or  even  six  smaller  branches 
given  off  from  a  common  base. 

In  the  lower  incisors,  cuspids,  and  bicuspids,  a  single  root  is 
most  frequently  met  with,  although  the  latter  class,  in  rare  in- 
stances, may  be  provided  with  two.  The  lower  first  and  second 
molars  are  each  provided  with  two  roots,  but  in  the  third  molar, 
like  its  superior  fellow,  the  number  may  be  diminished  or  in- 
creased. In  the  upper  molars,  two  of  the  three  roots  are  placed 
above  the  buccal  half  of  the  crown,  and  are  called  buccal  roots  ; 
the  remaining  root  is  placed  above  the  lingual  half  of  the  crown, 
and  is  designated  as  the  lingual  root.  In  the  lower  molars,  one 
of  the  two  roots  is  placed  below  the  anterior  or  mesial  half  of 
the  crown,  and  is  named  the  mesial  root,  and  the  other  below 
the  posterior  or  distal  half,  and  is  known  as  the  distal  root.  In 
those  teeth  with  a  complicated  root  formation,  it  would  seem  to 
be  a  question  whether  they  are  possessed  of  a  single  root,  with 
two  or  more  branches,  or  separate  and  distinct  roots  throughout. 
To  determine  this,  some  account  must  be  taken  of  the  point  at 
which  the  bifurcation  or  trifurcation  takes  place.  If  this  separa- 
tion be  in  close  proximity  to  the  crown,  the  tooth  should  be 
considered  as  having  more  than  one  root  (Fig.  30,  A) ;  but,  on 
the  other  hand,  if  the  point  of  separation  be  some  distance  from 
the  crown,  with  a  solid  mass  of  root  substance  intervening,  the 
tooth  may  be  said  to  possess  a  single  root,  with  two  or  more 


A   GENERAL   DESCRIPTION   OF   THE   TEETH. 


branches  (Fig.  30,  B).  In  the  latter  instance,  that  part  of  the 
tooth  between  the  point  of  separation  and  the  crown  is  called 
the  root  or  root  base  ;  while  the  prolongations  beyond  the  point 
of  separation  are  known  as  the  branches  of  the  root. 

The  roots  of  the  teeth  are  not  only  variable  in  number,  but 
are  also  subject  to  much  variety  in  form.  In  the  antei-ior  teeth 
(the  incisors  and  cuspids)  the  roots  are  inclined  to  the  form  of 
the  simple  cone,  which  form,  however,  is  frequently  more  or  less 
broken  by  a  slight  curvature 
near  their  extremities,  or  by 
a  slight  compression  of  their 
lateral  walls.  In  the  poste- 
rior teeth  (the  bicuspids  and 
molars)  the  roots,  root 
bases,  or  root  branches  are 
all  inclined  to  the  conical 
form,  but  do  not  approach 
so  nearly  the  perfect  cone 
as  those  of  the  anterior 
teeth.  These  roots  are  also 
more   or   less    crooked    and  "         p,^   ,       -^ 

ric.   jO. 

flattened  laterally.    The  free 

extremity  of  the  roots  of  the  teeth,  forming-  as  they  do  the  apex 
of  these  cone-like  prolongations  of  the  crowns,  are  known  as 
the  apices  or  apical  extremities. 

The  extent  of  the  enamel  covering  to  the  crowns  of  the  teeth 
is  marked  by  a  well-defined  line,  which  completely  encircles  the 
neck  of  the  tooth,  the  cervical  line. 

The  Dental  Arch  (Fig.  31). — The  teeth  are  arranged  in  the 
jaws  in  the  form  of  two  parabolic  curves,  the  superior  arch  de- 
scribing the  segment  of  a  larger  circle  than  the  inferior,  as  a  result 
of  which  the  upper  teeth  slightly  overhang  the  lower.  Figure 
31  represents  the  sixteen  upper  teeth  in  position  in  the  bone, 
presenting  their  occlusal  surfaces,  a  part  of  their  lingual  sur- 
faces also  being  visible.  Viewed  in  this  direction,  the  gradual 
change  in  the  crowns  of  the  teeth  from  the  simple  incisors  to 
the  complex  molars  may  be  observed.  An  examination  of  the 
central  incisors  will  show  how  perfectly  they  are  adapted  to  the 


process  of  cutting  or  incising  the  food,  the  cutting-edge  being 
sharp  and  the  lingual  surface  comparatively  smooth  and 
unbroken.  In  the  lateral  incisors  the  cutting  feature  predomi- 
nates, but  the  lingual  surface  is  broken  near  the  neck  of  the 
tooth  by  a  slight  depression,  surmounted  by  a  more  or  less  pro- 
nounced fold  of  enamel,  in  many  instances  resembling  a  small 
cusp.  The  crown  of  the  cuspid  tooth  furnishes  the  intermediate 
form  between  the  simple  and  the  complex.  This  tooth,  instead 
of  being  provided  with  a  straight  cutting-edge,  is  surmounted 


Fig.  31. — The  Dental  Arch. 


at  the  center  of  its  occlusal  surface  with  a  well-defined  point  or 
cusp,  descending  from  the  summit  of  which  are  two  cutting- 
edges,  one  passing  to  the  mesial  and  one  to  the  distal.  The 
lingual  surface  of  this  tooth  presents  a  marked  contrast  to  the 
corresponding  surface  of  the  incisors,  being  broad  and  full,  and 
frequently  provided  with  a  prominent  ridge  of  enamel  in  the 
region  of  the  neck,  showing  a  rapid  approach  to  the  complex 
form.  In  the  bicuspids  the  buccal  half  of  the  crown  is  quite 
similar  to  the  crown  of  the  cuspid,  but  in  the  lingual  half  a  com- 
plete revolution  has  taken  place.    The  enamel  fold — but  slightly 


A   GENERAL   DESCRIPTION   OF   THE   TEETH.  loi 

apparent  in  the  incisors,  and  somewhat  increased  in  the  cuspids — 
has  now  become  a  fully  developed  cusp,  resulting  in  the  produc- 
tion of  an  occlusal  surface  adapted  to  crushing  or  grinding, 
instead  of  incising  or  tearing.  In  the  molars,  the  increase  in 
the  size  of  the  tooth-crown  is  accompanied  with  an  occlusal 
surface  much  more  complex  than  any  of  the  teeth  previously 
described,  and  one  well  adapted  to  its  function — that  of  crushing 
and  grinding  the  food. 

Arrangements  of  the  Teeth  in  the  Dental  Arch  (Fig.  31). — 
Beginning  with  the  upper  teeth,  the  central  incisors  are  found 
occupying  the  center  of  the  arch,  and  are,  therefore,  slightly  in 
advance  of  the  laterals.  These  teeth  are  so  implanted  in  the 
alveoli  that  their  crowns  are  not  perpendicular,  the  cutting  edge 
being  slightly  more  prominent  than  the  neck  of  the  tooth.  The 
roots  are  also  somewhat  inclined  from  the  median  line,  and  as  a 
result  the  crowns  have  a  slight  mesial  inclination,  the  mesial 
surfaces  approximating  each  other  at  or  near  the  cutting-edge, 
with  a  slight  space  intervening  at  the  necks.  In  certain  typal 
forms, — the  bilious,  for  example, — when  the  front  of  the  arch  is 
flat,  the  labial  surfaces  of  these  teeth  form  nearly  a  direct  line 
from  side  to  side  ;  while  in  those  types  in  which  the  arch  is  well 
rounded  anteriorly,  notably  in  the  sanguine  temperament,  the 
labial  surfaces  of  these  two  teeth  form  a  small  segment  of  the 
arch,  so  that  the  mesial  extremity  of  the  cutttng-edge  of  each 
tooth-crown  is  somewhat  in  advance  of  the  distal.  The  lateral 
incisors  are  similarly  implanted  in  the  alveoli,  causing  their  cut- 
ting-edges to  project.  The  roots  of  the  lateral  incisors  usually 
have  a  stronger  distal  inclination  than  those  of  the  centrals,  and 
the  crowns  show  a  more  marked  mesial  inclination.  The  mesial 
surfaces  approximate  the  distal  surfaces  of  the  central  incisors  at 
or  near  the  cutting-edge.  When  the  front  of  the  arch  is  flattened, 
these  teeth  are  but  little  less  prominent  than  the  central  incisors, 
but  when  the  arch  is  well  rounded  they  continue  the  segment  be- 
gun by  the  centrals,  and  are  necessarily  less  prominent.  While 
the  occlusal  surfaces  of  the  teeth  are  usually  considered  as  form- 
ing a  perfect  plane  (see  Occlusion  of  the  Teeth),  the  lateral  inci- 
sors are  generally  a  trifle  shorter  than  the  centrals.  The  cuspids 
may  be  considered  as  occupying  the  corners  or  turning-points 


of  the  arch.  They  are  more  prominently  placed  than  the 
adjoining  teeth,  this  feature  being  increased  by  the  bulging  or 
general  convexity  of  their  labial  surfaces.  The  extremity  of  the 
occlusal  surface  of  the  cuspids—/',  e.,  the  point  of  the  cusp — is 
a  trifle  below  the  cutting  edge  of  the  laterals  and  about  on  a 
line  with  that  of  the  centrals.  While  the  apical  extremities  of 
the  roots  of  the  cuspid  teeth  are  directed  away  from  the 
median  line,  the  crowns  assume  almost  a  perpendicular,  this 
condition  resulting  from  a  bend  in  the  tooth  at  the  neck. 
Although  the  perpendicular  position  is  most  commonly  assumed 
by  the  crown,  it  is  not  unusual  to  find  either  a  mesial  or  distal 
inclination  present.  Reference  has  been  made  to  the  cuspid 
teeth  occupying  a  position  which  might  be  termed  the  turning- 
points  or  corners  of  the  arch,  and  in  most  instances  it  may  be 
thus  considered  ;  but  in  certain  typal  forms — the  sanguine,  for 
example — the  tooth-line  is  unbroken  and  passes  over  the  cutting- 
edges  of  the  incisors,  the  summit  of  the  cusps  of  the  cuspids, 
and  is  continued  backward  over  the  buccal  cusps  of  the  posterior 
teeth.  The  bicuspids  are  placed  nearly  perpendicular  in  the 
arch,  but  occasionally  deviate  from  this  by  a  slight  mesial  or 
buccal  inclination.  The  length  usually  corresponds  to  that  of 
the  central  incisors,  and  their  buccal  surfaces  are  slightly  less 
prominent  than  the  corresponding  surfaces  of  the  cuspid  teeth. 
The  increase  in  the  buccolingual  diameter  of  the  crowns  of  the 
bicuspids  over  that  of  the  incisors  and  cuspids  results  in  break- 
ing the  lingual  line  of  the  occlusal  surfaces.  In  the  bilious  and 
kindred  types,  the  tooth-line  is  carried  directly  backward  from 
the  cuspid  to  the  first  molar,  making  the  buccal  face  of  the 
bicuspids  equally  prominent,  but  when  the  arch  is  well  rounded 
the  second  bicuspid  is  slightly  more  prominent  than  the  first. 
The  first  and  second  molars  usually  assume  a  perpendicular 
position,  but  are  occasionally  inclined  to  the  distal  and  buccal. 
The  relative  prominence  of  the  buccal  as  well  as  the  lingual 
surfaces  of  these  teeth  is  also  controlled  by  the  form  of  the  arch. 
The  occlusal  surfaces  are  about  on  a  level  with  those  of  the 
bicuspids  and  central  incisors,  but  generally  the  lack  of  devel- 
opment in  the  distal  half  of  the  crown  of  the  second  molar 
results  in  the  production  of  a  slight  upward  curve  to  the  tooth- 


A   GENERAL   DESCRIPTION    OF   THE   TEETH.  103 

line  level  at  this  point  (see  Occlusion  of  the  Teeth).  On  account 
of  the  limited  accommodations  afforded  it,  the  position  of  the 
upper  third  molar  is  quite  variable.  It  may  be  either  to  the 
buccal  or  to  the  lingual  of  the  tooth-line,  and  is  usually  strongly 
inclined  to  the  distal.  In  those  cases  in  which  there  is  a  decided 
dip  to  the  arch  (see  Occlusion  of  the  Teeth),  this  tooth  is  rela- 
tively shorter  than  those  anterior  to  it,  but  when  the  tooth-line 
level  is  a  perfect  plane,  the  length  of  this  tooth  corresponds  to 
the  other  molars  and  bicuspids. 

The  lower  incisors  are  placed  more  nearly  in  a  perpendicular 


Fig.  32. — The  Tooth-ltne  in  the  Lower  Jaw. 

position  than  the  upper,  and  a  reverse  condition  exists,  in  the 
lateral  incisors  being  a  trifle  larger  than  the  centrals.  The 
lower  cuspids  are  probably  more  constant  in  their  position  than 
any  class  of  teeth  in  the  mouth,  in  nearly  all  instances  assuming 
a  direct  perpendicular.  Like  the  upper  cuspids,  they  may  be 
said  to  establish  the  corners  or  turning-points  of  the  inferior 
arch,  and  are  somewhat  more  prominent  in  the  tooth-line  than 
neighboring  teeth.  All  of  the  six  anterior  lower  teeth  may  be 
slightly  inclined  to  the  mesial.  The  lower  bicuspids  and  molars, 
instead  of  having  the  buccal  inclination  possessed  by  the  corre- 
sponding  upper  teeth,   are   inclined   to  the  lingual.     The   first 


ANATOMY. 


molar  seldom  deviates  either  to  the  mesial  or  the  distal,  the 
second  molar  is  generally  inclined  to  the  mesial,  while  the  third 
molar  is  strongly  inclined  to  the  mesial.  In  the  inferior  arch 
the  curve  formed  by  the  incisors  and  cuspids  is  the  segment  of 
a  smaller  circle  than  the  corresponding  curve  in  the  superior 
arch.  This  curve  may  be  continued  over  the  buccal  cusps  of 
the  bicuspids  and  molars,  or  it  may  be  broken  at  the  cuspid 
tooth  and  continued  backward  in  a  direct  line  (Fig.  32).  The 
teeth  in  the  inferior  arch  are  placed  directly  over  the  body  of 
the  bone  as  far  back  as  the  second  bicuspids,  while  the  molars 


Fig.  33. — The  Tooth-line  in  the  Lower  Jaw. 

frequently  overhang  the  body  of  the  bone  by  an  extension  of 
the  alveoli  inward  (Fig.  33). 

The  curve  described  by  the  dental  arch  is  quite  variable,  and 
this  variation  is  generally  referred  to  in  connection  with  the 
temperament.  Thus,  in  the  sanguine  temperament  (Fig.  34),  the 
arch  is  well  rounded  anteriorly,  the  circle  being  continued  back- 
ward to  the  region  of  the  molars,  where  the  line  is  broken  by 
slightly  inclining  to  the  lingual.  In  this  arch  the  distance  in  a 
straight  line  from  the  center  of  the  second  molar,  on  one  side, 
to  the  center  of  the  corresponding  tooth  on  the  other,  is  about 
equal  to  the  distance  from  either  of  these  points  to  the  median 
line  between  the  central  incisors,  forming  a  right-angle  triangle. 


A   GENERAL   DESCRIPTION   OF   THE   TEETH.  105 

In  the  bilious  temperament  (Fig.  35)  the  arch  presents  a  broad 
front  from  cuspid  to  cuspid,  with  but  little  curve  ;  at  these  points 
it  turns  abruptly  backward,  being  continued  almost  in  a  direct 


_  Inff^y  |J.  '  '   :    -..^ 

i 

's 

I 

m€ik 

Ij 

H^^P' 

■-^ 

Fig.  34. — Sanguine. 


line  to  its  extremity.  In  this  arch  the  side  of  the  triangle  (repre- 
sented by  the  line  from  molar  to  molar)  is  much  reduced  in  length. 
In  the  nervous  temperament  (Fig.  36)  the  arch  is  Gothic  in  form, 


Fig.  35. — Bilious. 


the  segment  formed  by  the  anterior  teeth  being  that  of  a  much 
smaller  circle  than  either  of  the  types  previously  referred  to. 
The  distance  from  molar  to  molar  is  much  less  than  the  distance 


from   molar  to  median  line.      In  the  lymphatic  temperament 
(Fig.  37)  the  arch  is  well  rounded  and  broad,  the  segment  being 


fe. 

.^^  -^  ^^ir'' ' 

'B^fS^v/ifck 

,^0IS\4^  ^^K  V  : 

i 

M'iW^'''^ 

r^^f^y<;^B^ 

^ffir  v^JW^ 

^W^ 

Fig.  36. — Nervous. 

that  of  a  much  larger  circle  than  any  of  the  above,  the  side  of 
the  triangle  formed  by  the  line  from  molar  to  molar  being  of 
the  greatest  length. 


Fig.  37. — Lymphatic. 


Interproximate  Spaces. — In  the  mesiodistal  direction  the 
crowns  of  the  teeth,  as  a  class,  are  broader  at  their  occlusal 
surfaces  or  cutting-edges  than  at  their  necks  (Fig.  38).     This 


A   GENERAL   DESCRIPl'ION   OF   THE   TEETH.  107 

bell-shaped  form  of  the  tooth-crowns  causes  their  proximate 
surfaces  to  touch  at  a  point  representing  their  greatest  mesio- 
distal  diameter,  which  is  usually  near  the  cutting-edge  or  occlusal 
surface.  Between  this  point  of  contact  and  the  cervical  line 
there  exists  a  V-shaped  space,  called  the  interproximate  space. 
These  spaces  are  largest  in  that  class  of  teeth  found  in  the  ner- 
vous and  bilious  types,  where  the  necks  of  the  teeth  are  much 
constricted,   and  the  bell-shaped   crown  strongly  outlined.     In 


Fig.  38. — Section  of  the  Superior  Maxilla  Showing  Interpkoximate  Spaces. 

teeth  of  this  class  the  point  of  contact  is  slight,  and  the  inter- 
proximate spaces  are  only  partially  occupied  by  the  gum  tissue, 
leaving  a  free  passage  between  the  point  of  contact  and  the  gin- 
gival margins.  In  the  sanguine  and  lymphatic  temperaments 
the  proximate  surfaces  of  the  teeth  are  nearer  parallel  with 
one  another,  thus  making  the  point  of  contact  cover  a  greater 
extent  of  surface,  and  reducing  the  size  of  the  interproximate 
spaces. 


CHAPTER  V. 
OCCLUSION  OF  THE  TEETH. 


Fig.  39. — The  Teeth  in  Occlusion. 

As  Stated  elsewhere,  the  teeth  are  arranged  in  the  mouth  in 
the  form  of  two  parabohc  curves,  one  of  which  occupies  the 
upper  half  and  the  other  the  lower  half  of  the  cavity.  To  prop- 
erly perform  their  function  it  is  necessary  for  the  upper  and 
lower  teeth  to  come  in  contact,  which  they  are  enabled  to  do 
by  the  movement  of  the  lower  jaw,  and  it  is  the  relation  exist- 
ing between  the  two  when  thus  brought  together  that  consti- 
tutes the  occlusion  of  the  teeth.  During  mastication  the  teeth  do 
not  only  occlude,  and  remain  stationary  at  a  given  point  until 
the  lower  jaw  is  again  depressed,  but,  through  the  combined 
movements  of  the  mandible,  the  lower  teeth  are  made  to  move 
from  side  to  side,  thus  grinding  or  crushing  any  substance  placed 
between  the  occlusal  surfaces  of  the  bicuspids  and  molars.    This 


OCCLUSION   OF   THE   TEETH.  109 

gliding  antagonism  of  the  teeth  is  commonly  termed  the  articula- 
tion, and  it  is  important  that  a  distinction  be  made  between  the 
terms  "occlusion"  and  "articulation,"  the  former  referring  to 
the  relations  existing  between  the  upper  and  lower  teeth  when 
brought  together  normally  and  held  firmly  in  that  position, 
while  the  latter  relates  to  the  various  movements  of  the  teeth 
after  being  brought  together  in  occlusion.  In  the  majority 
of  instances  the  segment  described  by  the  superior  arch  is  some- 
what larger  than  that  formed  by  the  inferior,  and  the  upper 
teeth  project  over  and  are  pardy  outside  of  those  in  the  inferior 
arch.  Figure  39  presents  a  labial  and  buccal  view  of  the  teeth  in 
position  in  the  alveoli,  and  also  in  occlusion.  It  will  be  observed 
that  the  upper  teeth  are  not  directly  antagonistic  to  those  of 
the  same  name  in  the  inferior  arch.  There  are  two  reasons  for 
the  presence  of  this  condition  :  First,  the  mesiodistal  diameter 
of  the  upper  central  incisors  is  much  greater  than  that  of  the 
corresponding  lower  teeth  ;  second,  the  larger  circle  present 
in  the  superior  arch.  This  arrangement  provides  that  each 
tooth,  instead  of  being  antagonized  by  a  single  tooth  of  the 
opposite  jaw,  is  met  in  occlusion  by  a  portion  of  two  teeth. 
The  upper  central  incisor  is  met  in  occlusion  by  the  entire 
cutting-edge  of  the  lower  central  incisor  and  the  mesial  third 
of  the  cutting-edge  of  the  lower  lateral  incisor.  The  upper 
lateral  incisor  is  met  in  occlusion  by  the  distal  two-thirds  of  the 
cutting-edge  of  the  lower  lateral  incisor  and  by  the  mesial 
cutting-edge  of  the  lower  cuspid.  The  upper  cuspid  is  met 
in  occlusion  by  the  distal  cutting-edge  of  the  lower  cuspid 
and  by  the  mesial  two-thirds  of  the  buccal  cusp  of  the  lower 
first  bicuspid.  The  upper  first  bicuspid  is  met  in  occlusion 
by  the  remaining  or  distal  third  of  the  lower  first  bicuspid  and 
by  the  mesial  two-thirds  of  the  buccal  cusp  of  the  lower  second 
bicuspid.  The  upper  second  bicuspid  is  met  in  occlusion  by 
the  remaining  or  distal  third  of  the  buccal  cusp  of  the  lower 
second  bicuspid,  and  by  the  mesial  incline  of  the  mesiobuccal 
cusp  of  the  lower  first  molar.  The  upper  first  molar  is  met 
in  occlusion  by  the  distal  incline  of  the  mesiobuccal  cusp  of  the 
lower  first  molar,  by  the  entire  distal  cusp  of  the  same  tooth, 
and  by  the  mesial  incline  of  the  mesiobuccal  cusp  of  the  lower 


ANATOMY. 


second  molar.  The  upper  second  molar  is  met  in  occlusion 
by  the  distal  incline  of  the  mesiobuccal  cusp  of  the  lower 
second  molar,  by  the  entire  distobuccal  cusp  of  the  same  tooth, 
and  by  the  mesial  incline  of  the  mesiobuccal  cusp  of  the  lower 
third  molar.  The  upper  third  molar  is  met  in  occlusion  by 
the  distal  incline  of  the  mesiobuccal  cusp  of  the  lower  third 
molar  and  by  the  entire  distobuccal  cusp  of  the  same  tooth, 
thus  being  the  only  tooth  in  the  superior  arch  with  but  a 
single  opponent.  Likewise  each  lower  tooth  is  met  in  occlu- 
sion by  two  in  the  superior  arch,  with  the  single  exception 
of  the  central  incisor,  which  occludes  with  the  upper  central 


Fig.  40. — The  Mandible  at  the  Adult  Period,  showing  the  Equilateral  Triangle 

DESCRIBED    BY   THE   DeNTAL   ArCH. 


alone.  There  are  many  variations  from  this  so-called  typical 
occlusion,  as  above  described,  and  any  slight  difference  one  way 
or  another  should  not  be  considered  abnormal.  In  certain  types 
the  segmental  form  of  the  superior  arch  is  but  little  greater  than 
that  of  the  inferior,  and  the  cutting-edges  of  the  upper  incisors 
occlude  almost  directly  upon  the  cutting-edges  of  the  lower 
incisors.  As  a  result,  all  of  the  upper  teeth  are  forced  to  the 
distal,  and  the  relationship  between  the  upper  and  lower  organs 
is  much  altered.  When  the  upper  teeth  overhang  the  lower,  the 
lingual  cusps  of  the  upper  bicuspids  and  molars  penetrate  the 
fossae  or  sulci  of  the  corresponding  lower  teeth,  when  in  occlu- 


OCCLUSION  OF   THE   TEETH.  iii 

sion,  and  the  buccal  cusps  of  the  lower  bicuspids  and  molars 
rest  in  the  fossse  of  their  upper  opponents. 

To  assist  in  the  study  of  the  occlusion  of  the  teeth,  some  ref- 
erence must  be  made  to  the  tooth-line  level,  or  plane  of  oc- 
clusion. For  this  purpose  the  lines  forming  the  facial  angle 
are  of  value.  These  lines  are  as  follows  :  A  fixed  line  repre- 
senting the  base  of  the  angle  may  be  drawn  from  the  center  of 
the  glenoid  fossa,  passing  forward  through  the  anterior  nasal 
spine  or  base  of  the  nose  (a,  Fig.  41),  the  angle  being  com- 
pleted by  a  perpendicular  line  resting  upon  the  labial  surface  of 
the   upper    incisors,  passing    upward    and    touching    the    most 


/ 

""'X^ 

/ 

■^ 

/fe^ 

1 

^^ipr 

/ 

- 

Fig.  41. — Lines  showing  Facial  Angle,  Caucasian  or  White  Race. 


prominent  part  of  the  forehead  (b.  Fig.  41).  The  tooth-line 
level  is  approximately  horizontal  to  this  basal  line,  but  instead  of 
a  perfect  plane  we  usually  find  the  superior  arch  dipping  down- 
ward, while  the  inferior  arch  will  be  provided  with  a  correspond- 
ing depression.  This  dip  to  the  arch  is  greatest  in  the  region 
of  the  bicuspids,  and  the  extent  to  \vhich  it  may  exist  varies  with 
the  type  of  tooth  and  the  consequent  nature  of  the  occlusion. 

Thus  far  no  reference  has  been  made  to  what  is  commonly 
termed  the  overbite,  and  the  cusp  forms  in  the  teeth.  As 
these  two  factors  exert  a  dominating  influence  over  the  char- 
acter of  the  occlusion,  the  effects  which  they  produce  will 
be  briefly  described.     The  overbite  is  so  named  from  the  upper 


incisors  and  cuspids  projecting  beyond,  or  overhanging  and 
partly  covering,  the  labial  surfaces  of  the  corresponding  lower 
teeth.  This  may  be  a  pronounced  feature  in  the  tooth-occlu- 
sion, or  it  may  exist  to  a  very  slight  degree.  Although  the 
overbite  is  usually  referred  to  as  existing  in  the  incisive  region 
alone,  it  is  not  confined  to  these  teeth,  but  is  also  present  in  the 
bicuspids  and  molars  by  the  buccal  cusps  of  the  upper  teeth 
overhanging  those  of  the  lower.  The  extent  of  the  over- 
bite is  gradually  diminished  from  before  backward,  the  central 
incisors  presenting  the  greatest  amount  of  overhanging  surface, 


Fig.  42. — Lines  showing  Facial  Angle,  Negro  or  Mixed  Races. 


which  condition  is  slightly  decreased  in  the  laterals,  and  a  cor- 
responding reduction  is  continued  until  the  third  molars  are 
reached,  at  which  point  the  overbite  is  scarcely  observed. 
Where  the  overbite  is  extensive,  as  shown  in  figure  39,  the 
upper  incisors  overhanging  and  hiding  from  view  about  one- 
third  of  the  labial  surfaces  of  the  lower  incisors,  the  cusps  of 
the  bicuspids  and  molars  will  be  correspondingly  long  and  pene- 
trating, the  buccal  cusps  of  the  upper  teeth  extending  well 
down  over  the  buccal  cusps  of  the  corresponding  lower  teeth. 
In  an  occlusion  of  this  class,  which  is  usually  found  in  the  ner- 
vous and  bilious  types,  the  dip  to  the  arch  will  be  a  prominent 


OCCLUSION  OF   THE   TEETH. 


feature,  the  occlusion  will  be  firm  and  well  locked,  and  the  lateral 
articular  movements  will  be  slight  during  mastication.  In  the 
lymphatic  and  sanguine  temperaments  the  occlusion  is  loose  and 


Fig.  43. — Lymphatic. 


Fig.  44. — Sanguine. 


f    «    f     f 


Fig.  45. — Bilious. 


wandering,  greater  freedom  of  movement  being  permitted  by 
the  short  overbite  and  the  corresponding  lack  of  cusp-formation. 
Figure  43   represents  such  an  occlusion  ;  the  superior  arch  is 


but  little  greater  in  its  segmental  outline  than  the  inferior.  The 
cutting-edges  of  the  upper  incisors  are  somewhat  more  prom- 
inent than  those  of  the  lower,  but  the  former  do  not  overlap 
the  labial  surfaces  of  the  latter.  In  an  occlusion  of  this  charac- 
ter the  dip  to  the  arch  is  not  so  pronounced,  and  the  articular 
movements  are  much  more  extensive. 


CHAPTER  VI. 
THE  BLOOD-  AND  NERVE-SUPPLY  TO  THE  TEETH. 

THE  BLOOD-SUPPLY  TO  THE  TEETH. 

Briefly  stated,  the  course  of  the  blood  from  the  heart  to  the 
teeth  is  as  follows :  From  the  heart  to  the  aorta,  to  the  common 
carotid  artery,  to  the  external  carotid  artery,  to  the  internal 
maxillary  artery,  from  the  various  branches  of  which  the  teeth 
are  supplied. 

The  Internal  Maxillary  Artery  (Fig.  47). — This  artery, 
otherwise  known  as  the  deep  facial,  is  the  larger  of  the  two  ter- 
minal branches  of  the  external  carotid.  In  addition  to  supply- 
ing the  teeth,  it  is  distributed  to  the  roof  and  floor  of  the  mouth, 
to  the  maxillary  sinus,  and  to  other  parts  of  the  face  and  head. 
It  has  its  origin  from  the  external  carotid  artery  opposite  the 
condyle  of  the  mandible  within  the  substance  of  the  parotid 
gland,  passes  forward  between  the  condyle  of  the  jaw  and  the 
sphenomaxillary  ligament,  from  which  point  it  passes  obliquely 
upward  and  forward  between  the  external  and  internal  ptery- 
goid muscles  until  it  reaches  the  sphenomaxillary  fossa,  where 
its  terminal  branches  are  given  off.  It  is  divided  into  three  por- 
tions— the  first  or  maxillary,  the  second  or  pterygoid,  and  the 
third  or  sphenomaxillary.  The  teeth  are  supplied  from  branches 
of  the  first  and  third  divisions,  the  upper  teeth  receiving  their 
blood-supply  from  the  alveolar  or  superior  maxillary  and  the 
infra-orbital  branches  of  the  third  division,  while  the  lower  teeth 
are  supplied  by  the  inferior  dental  branch  of  the  first  division. 

The  alveolar  or  superior  maxillary  branch  arises,  in 
common  with  the  intra-orbital  branch,  from  the  internal  maxillary 
as  it  passes  into  the  sphenomaxillary  fossa.  It  passes  down- 
ward, in  a  tortuous  manner,  in  a  groove  provided  for  it  in  the 
back  of  the  maxilla.  In  its  downward  course  it  gives  oft  the 
following  bi-anches :  The  antral,  to  supply  the  antrum;  the 
dental  (known  as  the  posterior  dental  arteries),  which  pass  into 


Fig.  46. — Dissection  Showing  Blood-supply  to  the  Teeth. 


THE   BLOOD-SUPPLY   TO   THE   TEETH.  117 

the  substance  of  the  bone  through  the  posterior  dental  canals 
to  supply  the  molar  and  bicuspid  teeth;  the  alveolar  or  gingival, 
to  supply  the  gums  ;  and  the  buccal,  to  the  lateral  walls  of  the 
mouth.  The  an-terior  upper  teeth  are  supplied  through  the 
infra-orbilal  branch  of  the  internal  maxillary.  This  branch  arises 
from  the  internal  maxillary  artery,  generally,  in  common  with  the 
posterior  dental.      It  passes  forward  in  company  with  the  maxil- 


Inf  aorbi  al  aitny  and  nene 


Spheno-palahne  branch 

Posterior  01  descending  palatine  branch 
^aso-patatme  blanch 

Vidian  branch 

Antei  cor  deep  temporal  artery 

Ezlemal  pterygoid  branch 


Orbital  branch 


Nasal  branch 

Anterior 

dental  branch 

Labial  branch 

Posterior  dental 

branch 


,  I         /  Posterior  deep  temporal  artery 

\\     I'/    '     /  /  ^  Small  1 


Submental  branch 


Bvecal  branch  : 
portion  0/  buccal  nervt 
Mylo-hyoidean  branch 


Internal  pterygoid  branch 


Fig.  47. — Scheme  of  Internal  Maxillary  Artery. 


lary  division  of  the  fifth  nerve — first  along  the  groove  and  then 
in  the  canal  on  the  orbital  plate  of  the  maxilla,  and  finally  makes 
its  exit  upon  the  face  through  the  infra-orbital  foramen.  Besides 
giving  off  branches  to  the  orbital  and  nasal  cavities,  it  supplies 
the  incisor  and  cuspid  teeth  through  its  anterior  dental  branch, 
which  passes  downward  through  a  groove  in  the  anterior  wall 
of  the  maxilla. 


Fig.  48.— Dissection  Showing  Nerve-supply  to  the  Teeth. 


THE   BLOOD-SUPPLV   TO   THE   TEETH.  119 

The  Inferior  Dental  Artery  (Fig.  47). — The  lower  teeth 
receive  their  blood-supply  through  the  inferior  dental  or 
mandibular  artery.  This  artery  arises  from  the  under  part  of 
the  internal  maxillary  as  it  passes  downward  and  forward 
between  the  sphenomaxillary  ligament  and  the  neck  of  the  jaw 
and  enters  the  inferior  dental  canal  through  the  inferior  dental 
foramen.  It  passes  forward  in  the  canal  accompanied  by  the 
inferior  dental  nerve,  and  in  so  doing  sends  off  twigs  to  supply 
the  molar  and  bicuspid  teeth.  When  the  mental  foramen  is 
reached,  it  divides  into  two  brandies,  the  incisive  branch  and  the 
mental  branch. 

The  incisive  branch  continues  its  course  within  the  cancellated 
structure  of  the  bone,  sending  off  minute  branches  which  supply 
the  anterior  teeth,  the  terminal  branches  anastomosing  with  the 
artery  of  the  opposite  side. 

The  mental  branch  passes  out  through  the  mental  foramen 
accompanied  by  the  mental  branches  of  the  inferior  dental  nerve, 
and  supplies  the  tissues  of  the  chin  and  lower  lip. 

The  Veins. — The  blood,  in  returning  from  the  teeth  to  the 
heart,  is  first  taken  up  by  the  posterior  dental  and  inferior  dental 
veins,  which  in  their  course  follow  closely  that  of  their  cor- 
responding arteries.  These  veins,  in  conjunction  with  others 
which  accompany  branches  of  the  internal  maxillary  arterj^ 
form  the  pterygoid  plexus.  At  the  posterior  confluence  of  this 
plexus  the  returning  blood  empties  into  the  internal  maxillary 
vein.  Accompanied  by  the  internal  maxillary  artery  it  passes 
backward  and  outward,  enters  the  parotid  gland,  and  finally 
empties  into  the  temporomaxillary  vein  midway  between  the 
zygoma  and  the  angle  of  the  jaw.  After  leaving  the  substance 
of  the  parotid  gland,  the  temporomaxillary  vein  passes  down- 
ward until  near  the  angle  of  the  jaw,  where  it  divides  into  two 
branches,  one  of  which  passes  downward  and  slightly  forward, 
uniting  with  the  facial  to  form  the  common  facial  vein,  and  the 
other,  after  passing  downward  and  backward,  empties  into  the 
external  jugular  vein.  The  external  jugular  vein  returns  the 
principal  portion  of  the  blood  from  the  teeth,  and  from  its  point 
of  beginning  it  passes  almost  perpendicularly  downward  and 
empties  into  the  subclavian  vein,  which,  by  joining  with  the  in- 


ternal  jugular  vein,  forms  the  innominate  vein,  which,  in  turn, 
empties  into  the  superior  vena  cava,  thus  communicating  with 
the  heart. 

THE  NERVE-SUPPLY  TO  THE  TEETH. 

The  nerves  supplying  the  teeth  are  derived  from  branches 
of  the  fifth  cranial  nerve,  otherwise  known  as  the  trifacial  or 
trigeminal  nerve.  The  fifth  nerve  is  the  largest  of  the  cranial 
nerves,  and  consists  of  two  parts,  a  large  root  (sensory)  and 
a  small  root  (motor).  The  larger  portion  passes  into  a  ganglion 
(the  Gasserian  ganglion),  frequently  compared  to  the  ganglion 
on  the  posterior  root  of  the  spinal  nerve.  It  arises,  or  makes 
its  appearance,  at  the  surface  of  the  brain,  on  the  anterior  part 
of  the  side  of  the  pons  Varolii.  The  sensory  root,  which, 
through  its  branches,  supplies  the  teeth,  is  composed  of  from 
80  to  100  filaments,  each  inclosed  in  a  neurilemma,  the  entire 
bundle  being  bound  together  in  a  single  nerve. 

The  fifth  nerve  is  divided  into  three  divisions  :  First,  or  opJi- 
tJialmic ;  second,  or  supejHor  maxillary ;  and  third,  or  inferior 
maxillary  (mandibular).  The  branches  which  supply  the  teeth 
are  included  in  the  second  and  third  divisions,  the  upper  teeth 
being  supplied  by  branches  from  the  superior  maxillary  nerve, 
and  the  lower  teeth  by  branches  from  the  inferior  maxillary 
nerve. 

The  Second  Division,  or  Superior  Maxillary  Nerve  (Fig. 
49). — This  nerve,  composed  entirely  of  sensory  fibers,  is  inter- 
mediate in  size  between  the  inferior  maxillary  and  the  ophthal- 
mic divisions.  It  passes  forward  from  the  Gasserian  ganglion 
and  leaves  the  cranium  through  the  foramen  rotundum.  It 
traverses  the  upper  part  of  the  sphenomaxillary  fossa,  and 
passes  into  the  orbit  through  the  sphenomaxillary  fissure  ;  then 
passes  forward  along  the  infra-orbital  groove,  and  enters  the 
infra-orbital  canal,  where  it  receives  the  name  of  the  infra-orbital 
nerve.  Passing  through  this  canal,  it  emerges  upon  the  face 
through  the  infra-orbital  foramen.  The  superior  maxillary 
nerve,  beside  supplying  the  teeth,  sends  ofif  branches  to  the  dura 
mater,  to  the  orbit,  and  terminal  branches  in  three  groups — 
labial,  nasal,  and  palpebral.     The   branches   given   off"  to   the 


THE   NERVE-SUPPLY   TO   THE   TEETH.  121 

teeth  are  the  posterior  superior  denial,  the  middle  superior  dental, 
and  the  anterior  superior  dental. 

The  posterior  superior  dental  arises  from  the  second  division 
of  the  fifth  nerve,  by  one  or  two  roots,  just  before  it  passes  into 
the  infra-orbital  canal.  It  is  divided  into  a  superior  and  an  infe- 
rior set ;  the  former  passes  forward  and  terminates  in  the  canine 
fossa,  while  the  latter,  usually  the  larger,  enters  the  posterior 
dental  canals,  and,  following  the  line  of  the  alveolar  process 
through  minute  canals  in  the  bone,  sends  off  twigs  to  the  molar 
teeth,  ending  in  a  plexiform  manner  by  communicating  with  the 


AyiERIOR  DENIAL       MAXILLARY  NEEVB        ORBITAL  URAACH 


MAXILLARY  KEEVE 


MECKEL'S  GANGLION 


POSTERIOR  DENTAL 


LOOP  FORMED  BY  MIDDLE  AND  ANTERIOR  DENTAL  NERVES 

Fig.  49. — The  Maxillary  Nerve,  Seen  from  Without. — {Morris,  afia-  Beaunis.) 


middle  superior  dental  nerve.  This  nerve  is  also  distributed  to 
the  gums  and  adjacent  buccal  mucous  membrane. 

Middle  Siiperior  Dental  Nerve. — The  infra-orbital  nerve,  soon 
after  entering  its  canal,  gives  off  this  branch,  which  passes  out- 
ward, downward,  and  forward  over  the  outer  wall  of  the  maxil- 
lary sinus,  and,  after  forming  plexuses  with  the  posterior  dental 
branches,  gives  off  filaments  to  supply  the  bicuspid  teeth. 

Tlie  anteHor  superior  dental  nerve,  which  is  the  largest  of  the 
dental    set,  is  given  off  from   the  infra-orbital   nerve,  enters  a 


canal  close  to  the  infra-orbital  foramen,  passes  over  the  anterior 
wall  of  the  maxillary  sinus,  and,  after  communicating  with  the 
middle  and  posterior  dental  nerves,  divides  into  ascending  and 
descending  branches,  the  latter  being  distributed  to  the  incisor 
and  cuspid  teeth. 

The  Third  Division,  or  Inferior  Maxillary  Nerve  (Fig. 
50). — This  is  the  largest  of  the  three  divisions  of  the  fifth  nerve. 

Temporal  Bone 


Fig.  50.— Dissection  showing  Mandibular  (Third)  Division  of  Fifth  Nerve. 

and  is  both  motor  and  sensory  in  its  function.  Besides  being 
distributed  to  the  lower  teeth,  it  sends  filaments  to  the  lower 
portion  of  the  face,  the  muscles  of  mastication,  the  tongue,  and 
mandible.  It  arises  from  the  Gasserian  ganglion,  passes  down- 
ward, and  emerges  from  the  skull  through  the  foramen  ovale, 
after  which  it  divides  into  a  small  anterior  (motor)  branch  and 
a  large  posterior  (sensory)  branch. 

The  Inferior  Dental  Nerve. — This  is  the  largest  branch  of  the 


THE   NERVE-SUPPLY   TO   THE   TEETH.  123 

inferior  maxillary  nerve.  From  its  point  of  origin  it  passes 
downward  internally  to  the  external  pterygoid  muscle,  and,  upon 
reaching  a  point  between  the  ramus  of  the  mandible  and  the 
sphenomandibular  ligament,  it  enters  the  inferior  dental  canal 
through  the  posterior  or  inferior  dental  foramen.  Before  enter- 
ing the  foramen,  two  branches  are  given  off,  a  lingual  and  a  mylo- 
hyoid branch.  The  nerve  is  accompanied  through  the  inferior 
dental  canal  by  the  inferior  dental  artery,  and,  when  the  mental 
foramen  is  reached,  it  terminates  by  dividing  into  an  incisive  and  a 
mental  branch.  Between  the  posterior  dental  foramen  and  the 
mental  foramen  the  nerve  gives  off  a  series  of  twigs  to  the  bicuspid 
and  molar  teeth,  and  these,  by  communicating  with  one  another 
within  the  substance  of  the  bone,  form  a  fine  plexus. 

The  incisive  branch  follows  the  incisive  arteries  through  the 
substance  of  that  part  of  the  bone  between  the  mental  foramen 
and  the  symphysis,  and  supplies  the  incisor  and  bicuspid  teeth, 
while  the  mental  branch  passes  forward  to  supply  the  chin  and 
lower  lip. 


CHAPTER  VII. 


OTHER      STRUCTURES     WITHIN      THE      MOUTH.— THE      GUMS.— THE 

MUCOUS   MEMBRANE.— THE  ALVEOLODENTAL   MEMBRANE.— 

GLANDS,   DUCTS,   ETC. 


Fig.  51. — Section  through  Gum  and  Alveolus. 

The  Gums  {Gingivce). — The  gums  are  formed  by  a  layer  of 
tough  fibrous  vascular  tissue,  covering  the  alveoli,  closely  attached 
to  their  periosteum,  and  provided  with  a  free  margin  (gingival 
margin),  which  is  closely  molded  to  the  necks  of  the  teeth. 
They  are  covered  on  both  aspects  by  the  general  mucous  mem- 
brane of  the  mouth,  that  overlying  the  labial  and  buccal  surfaces 
being  reflected  from  the  lips  and  cheeks,  the  palatal  surface  being 
continuous  with  that  of  the  hard  palate,  and  the  lingual  surface 
reflected  from  the  under  surface  of  the  tongue  and  floor  of  the 

124 


THE   GUMS.  125 

mouth.  In  the  immediate  region  of  the  neclvs  of  the  teeth  the 
gums  are  especially  thin  and  hard,  being-  closely  adherent  to  the 
periosteum  and  alveolodental  membrane  in  this  region.  In  pass- 
ing toward  the  base  of  the  alveolar  walls  the  tissue  becomes  less 
firmly  attached  to  the  underlying  structure,  and,  when  finally 
passing  into  the  mucous  membrane  of  the  cheeks  and  lips,  is 
quite  loose  and  flabby.  This  condition  also  prevails  on  the 
lingual  aspect,  but  the  palatal  surface  remains  firm  throughout, 
the  entire  mucous  membrane  overlying  the  roof  of  the  mouth 
being  similar  in  structure  and  attachment  to  that  portion  imme- 
diately surrounding  the  necks  of  the  teeth.  In  various  situations 
about  the  labial,  buccal,  and  lingual  surfaces  of  the  gums  small 
slender  folds  of  mucous  membrane  are  found  extending  to  the 
surrounding  tissues.  These  folds,  which  act  as  a  bridle  or  curb 
to  the  adjacent  movable  parts,  are  known  as  the  frena  of  the 
mouth.  The  principal  frena  are  found  at  the  median  line,  and 
are  three  in  number — the  frenum  labium  superioris,  frenum 
labium  inferioris,  and  the  frenum  linguae.  The  two  former 
extend  from  the  inner  surface  of  the  lips  to  the  gums,  to  which 
their  extent  of  attachment  is  somewhat  variable.  The  frenum 
labium  superioris  is  usually  much  larger  than  the  frenum  labium 
inferioris,  and  its  attachment  to  the  gum  frequently  extends 
almost  to  the  gingival  border.  The  frenum  linguae  extends  from 
the  under  surface  of  the  tip  of  the  tongue  to  the  lingual  surface 
of  the  inferior  gums.  This  is  a  much  stronger  fold  than 
those  connected  with  the  lips.  Similar  bridles  are  found  in 
the  buccal  region,  usually  near  the  bicuspid  teeth,  but  they  are 
much  smaller  than  those  at  the  median  line.  The  gingival 
margins,  or  that  portion  of  the  gums  embracing  the  necks  of  the 
teeth,  present  much  variety  in  outline.  Instead  of  encircling 
the  neck  of  the  tooth  in  a  direct  line,  the  margins  are  made 
up  of  a  series  of  semicircles.  Using  the  incisive  region  for 
reference,  the  labial  and  palatal  margins  are  concave  rootward, 
while  the  interproximate  spaces  are  partly  or  completely 
filled  by  gum  tissue,  having  the  outline  reversed  or  convex 
in  the  direction  of  the  crowns  of  the  teeth.  The  gingival 
margin  is  also  termed  the  "free  margin  of  the  gum,"  this  name 
better  describing"  its  extent.     As  previously  stated,  the  gums  are 


attached  to  the  periosteum  and  peridental  membrane,  but  in 
most  instances,  and  particularly  before  the  adult  period,  the 
margins  of  the  gums  extend  beyond  the  alveolodental  membrane, 
the  limit  of  which  is  formed  by  the  cervical  line.  That  portion 
of  the  gum  margin  beyond  the  cervical  line  is  in  close  contact 
with  the  neck  of  the  tooth,  but  is  not  adherent  to  it,  the  connect- 
ing medium,  the  alveolodental  membrane,  not  being  present  to 
form  the  attachment.  The  curvature  of  the  gingival  margins, 
and  the  nature  of  the  tissues  which  enter  into  their  construction, 
are  usually  considered  as  strongly  indicative  of  the  temperament 
of  the  indivadual.  Thus,  in  the  bilious  temperament  the  mar- 
gins are  inclined  to  angularity  and  the  tissues  rather  thick  and 
firm.  In  the  sanguine  type  the  outline  formed  is  almost  a  per- 
fect semicircle,  and  the  tissues  are  of  moderate  thickness  and 
firmness.  In  the  nervous  type  the  curvature  is  strongly  par- 
abolic, and  the  tissues  firm  and  delicate.  In  the  lymphatic  the 
tissues  are  loose  and  thick,  and  the  curvature  is  long  and  poorly 
defined.  In  some  instances  the  interproximate  spaces  are  com- 
pletely filled  by  the  gingivae  ;  in  others  the  space  is  only  partly 
occupied  by  these  tissues.  The  former  condition  is  present  when 
the  proximate  surfaces  of  the  teeth  are  nearly  or  quite  parallel 
with  each  other,  thus  reducing  the  capacity  of  the  space.  The 
latter  condition  is  present  when  the  crowns  of  the  teeth  are 
bell-shaped  and  the  interproximate  spaces  extensive.  The  labial 
and  buccal  surfaces  of  the  gums  are  more  or  less  broken  by 
numerous  prominences  and  depressions,  all  of  which  accord 
with  the  variations  upon  the  surface  of  the  bone  beneath. 

Mucous  Membrane  of  the  Mouth. — The  term  "membrane"' 
in  a  general  sense  is  one  applied  to  thin  layers  of  tissue,  some- 
what elastic  and  of  a  whitish  or  reddish  color.  Such  tissues 
are  found  lining  either  closed  cavities  or  canals  which  open 
externally,  absorbing  or  secreting  fluids,  and  enveloping  various 
organs.  The  simple  membranes  are  of  three  varieties,  being 
either  mucous,  serous,  or  fibrous.  The  mucous  membranes  are 
so  called  from  the  clear  viscid  fluid  (mucus)  which  they  secrete. 
They  line  the  various  cavities  or  tracts  of  the  body  which  com- 
municate with  the  exterior.  The  three  grand  divisions  of 
mucous  membrane  are  those  lining  the  digestive,   respiratory, 


MUCOUS   MEMBRANE   OF   THE   MOUTH.  127 

and  genito-urinary  passages.  Lining  the  entire  cavity  of  tlie 
mouth  we  find  the  beginning  of  the  digestive  tract,  being  con- 
tinuous with  the  skin  on  the  exterior  and  performing  many 
similar  functions  within.  It  is  soft,  smooth,  and  velvety,  of  a 
bright  red  color,  and  quite  vascular  ;  it  is  covered  on  the  ex- 
terior by  a  layer  of  epithelial  cells  overlying  the  vascular  parts. 
Immediately  beneath  this  is  a  network  of  fibrous  connective 
tissue  forming  the  proper  mucous  membrane,  and  still  deeper 
is  a  third  layer,  somewhat  loose  in  texture,  but  composed  of 
fibrous  connective  tissue,  the  submucous  membrane.  The  oral 
mucous  membrane,  at  its  point  of  beginning  on  the  contiguous 
surfaces  of  the  lips,  is  endowed  with  keen  sensibility  ;  it  is  dry, 
bright  red  in  color,  and  plentifully  supplied  with  vascular  papillae, 
in  many  of  which  are  sensory  nerve  terminals.  Distributed 
along  the  line  of  junction  between  the  integument  and  the 
mucous  membrane  are  numerous  sebaceous  follicles,  which, 
however,  are  devoid  of  hair-bulbs.  The  characteristic  dryness 
of  this  surface  gradually  becomes  changed  to  a  mucus-secreting 
one,  as  that  part  of  the  membrane  lining  the  interior  of  the  lips 
is  approached.  Distributed  over  the  surface  of  the  labial 
mucous  membrane  are  a  number  of  minute  openings,  the 
mouths  of  the  labial  glands,  which  lie  immediately  beneath 
the  membrane.  The  buccal  mucous  membrane,  or  that  lining 
the  cheeks,  is  similar  to  that  covering  the  internal  surface  of 
the  lips.  It  is  penetrated  at  various  points  by  the  mouths 
of  the  buccal  glands,  which,  in  general,  are  smaller  and  less 
numerous  than  the  labial  glands.  In  the  region  of  the  second 
molar  teeth  the  membrane  is  broken  by  four  or  five  openings 
of  larger  size,  which  communicate  with  the  molar  glands.  The 
mucous  membrane  covering  the  hard  palate  is  thick  and  firm, 
less  brilliant  in  color  than  that  covering  the  cheeks  and  lips, 
and  firmly  bound  down  to  the  periosteum.  Running  from 
before  backward  at  the  median  line,  the  membrane  is  formed 
into  a  slight  fold,  the  median  raphe,  while  near  the  anterior 
portion  of  the  palate  are  a  number  of  fantastically  arranged 
folds,  the  rug(g  (see  General  Description  of  the  Mouth). 
The  thin  but  rather  dense  fibrous'  aponeurosis  forming  the 
soft  palate  is  covered  anteriorly  by  the  oral  mucous  membrane. 


Suspended  from  the  center  of  the  free  margin  of  the  soft 
palate  is  the  uvula,  which  is  likewise  covered  by  mucous  mem- 
brane, and  from  the  base  of  this,  on  either  side,  are  two  folds 
of  the  membrane,  which  extend  outward  and  downward,  form- 
ing the  anterior  and  posterior  pillars  of  the  fauces.  The 
mucous  membrane  covering  the  tongue  has  already  been  de- 
scribed in  connection  with  that  organ  (see  p.  41).  From  the 
mouth  the  digestive  mucous  membrane  passes  through  the 
fauces,  pharynx,  and  esophagus  to  the  stomach,  and  is  so 
continued  throughout  the  whole  digestive  tract.  Other  pro- 
longations also  pass  into  the  ducts  of  the  salivary  glands.* 

The  Alveolodental  or  Root  Membrane. 

This  membrane  invests  the  roots  of  the  teeth,  and  at  the  same 
time  lines  the  wall  of  the  alveoli.  Being  reflected  from  the  peri- 
osteum covering  the  outer  alveolar  walls,  it  enters  the  alveolar 
sockets  as  a  single  membrane,  affording  nourishment  to  the  bone 
on  one  side  and  to  the  cementum  of  the  tooth  on  the  other. 
It  is  a  connective  tissue  of  moderate  density,  and  is  rich  in  its 
nerve-  and  blood-supply.  The  general  direction  of  its  fibers 
is  transverse,  being  attached  at  one  extremity  to  the  alveolar 
wall  and  at  the  other  to  the  cementum  of  the  root.  The  con- 
nective-tissue fibers  are  not  merely  attached  to  the  surface  of 
the  calcified  structure,  but  the  strength  of  this  attachment  is 
greatly  increased  by  the  passage  of  the  fibers  into  the  sub- 
stance of  the  bone  at  one  extremity  and  into  the  lamellse  ot 
the  cementum  at  the  other.  In  general,  the  membrane  is  more 
closely  adherent  to  the  cementum  than  to  the  bone,  usually 
clinging  to  the  former  when  removed  from  its  socket.  The  nature 
of  the  articulation  between  the  tooth-root  and  the  alveolar  socket, 
to  the  production  of  which  this  membrane  so  largely  contributes, 
is  one  peculiar  to  itself  While  there  is  no  marked  mobility, 
there  is,  nevertheless,  sufficient  elasticity  in  the  intervening  mem- 
brane to  provide  against  the  severe  concussions  and  lateral 
strains  incident  to  mastication,  the  former  being  provided  for  by 
the  general  elasticity,  while  the  latter  is  cared  for  by  specially 

*  For  a  minute  description  of  the  mucous  membrane  of  the  mouth  see  Part  ii. 


THE  ALVEOLODENTAL   PERIOSTEUM    OR   ROOT   MEMBRANE. 


distributed  fibers,  which  serve  to  return  the  tooth  to  its  normal 
position  when  sHghtly  rotated  or  laterally  displaced.  This  elas- 
ticity is  greatest  in  youth  and  up  to  the  meridian  of  life,  after 
which  time  it  gradually  becomes  less  pronounced.  The  membrane 
is  thickest  about  the  apical  ends  of  the  roots  and  in  the  cervical 
region,  and  the  distribution  of  the  fibers  at  these  points  is  some- 
what different  from  those  about  the  body  of  the  root.  In  the 
former  location  they  are 
spread  out  fan-like  from  the 
apical  root  surface  to  meet 
the  surrounding  alveolar 
wall,  while  in  the  latter  they 
pass  longitudinally  over  the 
alveolar  margins  to  unite 
with  the  periosteum  of  the 
parts.  In  conjunction  with 
the  functions  already  men- 
tioned, the  peridental  mem- 
brane is  the  medium  by 
which  all  forces  applied  to 
the  tooth-surface  are  taken 
up  and  conveyed  to  the 
brain.  A  tooth  in  a  normal 
condition  may  be  said  to 
possess  no  sense  of  touch, 
unless  it  be  so  severely  ap- 
plied as  to  make  its  influ- 
ence felt  by  the  pulp,  in 
which  case  the  sensation 
becomes  one  of  pain.  The 
nerves  of  the  membrane  act 
in  precisely  the  same  man- 
ner as  do  other  sensory-nerve  terminals,  being  influenced  by 
the  slightest  touch  applied  to  the  surface  of  the  tooth-crown.* 
In  certain  conditions  of  defective  hearing  the  alveolodental 
membrane    may  be  made  to    assist   this    function    by  the  use 


Fig.  52. — Root  and  Membrane  of  Tooth. 
p.,  /,   Peridental  membrane  ;  ap^  apical  space  ; 
a,   artery  ;    al,  al,  alveolar  process ;    /,   /,  dental 
ligament. 


■■  This  membrane  is  more  fully  described  in  Part  ii. 


I30  ANATOMY. 

of  an  instrument  made  for  the  purpose  known  as  a  denti- 
phone. 

Blood-supply  to  the  Alveolodental  Membrane. — A  very 
clear  Idea  of  the  blood-supply  to  this  membrane  may  be  obtained 
from  figure  52. 

Entering  the  alveolar  socket  as  a  single  arterial  branch,  the 
thickest  portion  of  the  membrane  is  gained  where  a  number 
of  smaller  twigs  are  given  off,  one  or  more  of  which  enter  the 
pulp-canal  of  the  tooth-root  through  the  apical  foramina  supply- 
ing the  pulp,  which  in  turn  supplies  the  tooth-structure  within, 
while  the  others  ramify  through  the  substance  of  the  alveolodental 
membrane,  through  its  capillaries,  supplying  the  cementum 
from  without;  while  passing  through  the  membranous  structure, 
further  minute  branches  are  given  off  which  penetrate  the  walls 
of  the  alveolus  and  anastomose  with  the  arteries  which  supply 
the  oral  mucous  membrane,  in  this  manner  providing  a  gen- 
erous blood-supply  to  the  parts.  Further  on  we  shall  see  that 
the  tooth-pulp  and  the  alveolodental  membrane  spring  from  the 
same  source  (see  Development  of  the  Teeth),  and  the  blood- 
supply  to  the  parts  during  the  saccular  stage  of  development 
is  alike  distributed  to  the  base  of  the  pulp  and  to  the  follicular 
walls.  After  the  completion  of  the  developmental  process  this 
distribution  is  but  little  changed,  the  blood-vessels  accommo- 
dating themselves  to  the  alterations  incident  to  the  generation 
of  the  parts. 

The  Nerve-supply  to  the  Alveolodental  Membrane. — 
The  nerve-supply  to  this  membrane  is  distributed  in  a  manner 
similar  to  the  blood-supply,  being  derived  from  a  single  filament 
given  off  from  the  dental  nerve  and  entering  the  tooth-socket 
by  the  side  of  the  blood-vessels,  and  by  numerous  filaments 
which  reach  the  structure  by  passing  through  the  many  minute 
canals  in  the  substance  of  the  alveolar  walls  and  the  intervening 
septa. 

GLANDS    OF    THE    MOUTH. 

The  glands  of  the  mouth  are  of  two  kinds,  being  either  serous 
or  mucous,  and,  as  they  differ  in  the  character  of  their  secretions, 
so  they  differ  in  structure.     The  mucous  glands  are  the  most 


GLANDS   OF  THE   MOUTH.  131 

numerous,  and  are  found  beneath  the  mucous  membrane  of  the 
lips,  in  the  same  membrane  lining  the  cheeks,  the  hard  and  soft 
palate,  the  tonsils,  and  at  the  back  of  the  tongue.  These  glands 
are  quite  variable  in  size,  but  are  all  of  macroscopical  propor- 
tions, appearing  when  examined  in  this  manner  as  minute 
whitish  bodies.  The  secretions  from  the  glands  are  poured  into 
the  mouth  through  small  ducts  which  pass  in  various  directions 
through  the  substance  of  the  mucous  membrane.  Beginning 
as  a  single  duct  for  each  gland,  they  pass  to  the  submucous 
tissue,  here  branching  into  two  or  more  smaller  ducts  termi- 
nating in  alveoli,  the  number  and  size  of  these  depending 
upon  the  size  of  the  gland  with  which  they  are  connected. 
The  glands  are  variously  named  according  to  their  location, 
those  occupying  the  lips  being  known  as  labial  glands ;  those 
of  the  cheeks,  the  buccal  glands ;  those  of  the  palate,  the  pal- 
atal glands,  etc.  The  mucous  glands,  although  differing  in  size, 
are  similar  when  histologically  considered. 

The  Labial  Glands. — These  are  among  the  largest  mucous 
glands  of  the  mouth,  and  are  more  numerous  in  the  upper  than 
in  the  lower  lip.  The  form,  size,  and  location  of  the  labial  glands 
may  best  be  studied  by  dissection,  which  may  readily  be  accom- 
plished by  first  removing  the  integument  and  muscular  tissues 
from  the  parts,  when  they  will  be  brought  into  view.  The  glands 
are  irregularly  arranged,  and  are  most  numerous  near  the 
median  line.  The  body  of  each  gland  is  rounded  and  held  in 
position  by  connective  tissue,  as  well  as  by  the  duct  connecting 
it  with  the  interior  of  the  mouth. 

Besides  the  mucous  glands  of  the  lips,  there  are  present 
numerous  sebaceous  glands.  These  are  somewhat  smaller,  and 
are  situated  beneath  the  mucous  membrane  covering  the  contigu- 
ous surfaces  of  the  lips,  the  numerous  ducts  leading  from  them 
opening  upon  these  parts. 

The  Buccal  Glands. — The  glands  of  the  cheek,  otherwise 
known  as  buccal  glands,  are  similar  to,  but  smaller  than  those  of 
the  lips,  and  are  placed  between  the  submucous  tissue  and  the 
buccinator  muscle.  These  glands  also  pour  their  secretions 
into  the  mouth  through  numerous  ducts  which  pass  through  the 
buccal  mucous  membrane. 


In  the  region  of  the  third  molar  teeth  another  set  of  mucous 
glands  open  into  the  mouth,  known  as  the  molar  glands.  They 
are  placed  between  the  buccinator  and  masseter  muscles,  are 
similar  in  construction,  and  secrete  a  like  fluid  to  those  pre- 
viously described,  being  larger  than  the  buccal  and  smaller 
than  the  labial  glands. 

Palatal  Glands. — Situated  between  the  mucous  membrane 
of  the  hard  palate  and  the  periosteum  are  numerous  mucous 
glands  similar  to  those  previously  described.  Provision  is  made 
for  the  accommodation  of  these  glands  by  many  small  depressions 
in  the  bony  plates  (see  Fig.  3).  They  are  irregularly  distributed 
over  the  surface  of  each  lateral  half  of  the  hard  palate,  but  are 
absent  at  the  median  raphe  and  immediately  beneath  the  rugse. 
The  mucous  membrane  of  the  hard  palate  is  tense  and  hard,  in 
consequence  of  which  it  is  not  so  thick  as  the  buccal  and  lingual 
membranes,  and  the  ducts  from  the  numerous  glands  are  there- 
fore not  so  long.  The  glands  of  the  soft  palate,  uvulae,  and 
fauces  are  situated  beneath  the  deep  layer  of  mucous  membrane 
covering  these  parts,  in  the  former  structure  opening  on  both 
the  oral  and  nasal  surfaces. 

Lfingual  Glands. — The  glands  of  the  tongue  are  of  two 
kinds — mucous  and  serous.  The  former  are  chiefly  found  at  the 
back  part  of  the  tongue,  but  a  few  of  smaller  size  are  present 
near  the  tip.  The  serous  variety  are  to  be  found  only  at  the 
back  of  the  organ,  and  are  closely  associated  with  the  taste 
organs  in  this  region.  These  glands  are  assisted  in  performing 
their  function  by  being  placed  between  bundles  of  striped  mus- 
cular tissue,  the  activity  of  which  forces  the  secretions  to  the 
surface  by  compressing  the  glands.  While  the  majority  of  the 
lingual  glands  are  present  in  the  circumvallate  region,  a  number 
are  found  distributed  beneath  the  mucous  membrane  of  the 
borders  and  tip  of  the  organ. 

The  Salivary  Glands  (Fig.  53). — These  glands,  while 
outside  the  mouth,  are  so  closely  associated  with  its  functions 
that  a  brief  description  will  be  presented.  The  chief  salivary 
glands  are  six  in  number,  three  on  each  side.  They  are  named 
parotid,  submaxillary,  and  sublingual ;  the  former,  secreting  true, 
thin,  watery  saliva,  is  a  true  salivary  gland,  while  the  latter  two 


GLANDS   OF   THE   MOUTH. 


133 


are  known  as   mixed,   or  mucosalivary  glands,   secreting   both 
mucus  and  saliva. 

The  Parotid  Gland. — This  gland  is  the  largest  of  the  three, 
and  is  placed  a  little  below  and  in  front  of  the  ear,  having  the 
following  boundaries  :  Anteriorly,  by  the  ramus  of  the  mandible  ; 
posteriorly,  by  the  styloid  and  mastoid  processes  of  the  temporal 
bone  ;  above,  by  the  root  of  the  zygoma,  and  below,  by  a  line 
drawn  backward  from  the  angle  of  the  jaw.     While  the  extent 


SOOIA  TAROTIDIG 


DUOT  OF  PAROTID 


Freenum  llaguee 
DUCT  OF  RIVINUS 


SUBLINGUAL  GLAND 


PAROTID  GLAND 


MaBBeter  muscle 


Mylo-hyoid  muBCle 


'/  Posterior  belly  of 

digastric  muscle 


Loop  of  fascia 


DEEP  PORTION  OF  SUBMAXILLARY  BLAND 

Fig.  53. — The  Salivary  Glands. — (Moi-ris.) 

and  outline  of  the  gland  is  somewhat  variable,  its  position  is 
approximately  that  oudined  in  figure  53.  Its  superficial  surface, 
somewhat  lobulated,  is  in  close  reladon  to  the  skin  and  fascia, 
while  deeply  it  penetrates  well  into  the  neck  by  two  processes, 
one  of  which  passes  behind  the  styloid  process  and  beneath  the 
mastoid  process  of  the  temporal  bone  and  the  sternomastoid 
muscle,  while  the  other  passes  in  front  of  the  styloid  process. 
Given  off  from  the  body  of  the  gland  and  extending  in  various 
directions  are  a   number  of  processes  or  lobes,  one  extending 


134  ANATOMY. 

forward  between  the  two  pterygoid  muscles,  and  known  as  the 
pterygoid  lobe ;  another  passing  into  the  glenoid  cavity,  the 
glenoid  lobe  ;  while  a  third  passes  deeply  between  the  carotid 
vessels,  and  is  called  the  carotid  lobe.  In  many  instances  there 
is  an  additional  lobe,  which  is  detached  from  the  body  of  the 
gland,  known  as  the  socia  parotidis.  When  present,  this  lobe  is 
placed  over  the  parotid  duct  and  empties  into  it.  Passing 
through  the  substance  of  the  gland  are  a  number  of  arteries  and 
veins,  principal  among  which  are  the  external  carotid,  transverse 
facial,  and  internal  maxillary,  the  gland  receiving  its  blood- 
supply  by  branches  from  these.  The  internal  carotid  artery 
and  internal  jugular  vein  lie  close  to  its  internal  surface.  The 
facial  nerve  and  its  branches  and  the  great  auricular  nerve  pass 
through  the  gland  from  before  backward,  and  supply  its  sub- 
stance with  nerve-force.  The  weight  of  this  gland  is  from  one- 
half  to  one  ounce. 

The  Parotid  Duct. — Leading  from  the  gland  to  the  mouth 
is  the  parotid  or  Stenson's  duct.  After  passing  through  the  fat 
of  the  cheek  and  the  fibers  of  the  buccinator  muscle,  the  duct 
comes  in  contact  with  the  deep  layer  of  the  oral  mucous  mem- 
brane. After  passing  between  this  structure  and  the  cheek 
tissues  for  a  short  distance  it  enters  the  mouth  opposite  the 
crown  of  the  upper  second  molar  tooth,  the  orifice  of  the 
duct  appearing  on  the  surface  of  the  mucous  membrane  in  the 
form  of  a  small  papilla,  which  may  be  readily  observed  with  the 
naked  eye.  When  first  given  off  from  the  gland  a  number  of 
small  ducts  are  present,  but  these  soon  unite  and  form  a  single 
canal.  The  parotid  duct  is  quite  dense,  of  considerable  thick- 
ness, and  is  lined  by  a  reflexion  of  the  buccal  mucous  mem- 
brane. 

The  Submaxillary  Gland. — This  gland,  which  receives  its 
name  from  occupying  a  position  below  the  maxillary  bone,  is 
somewhat  smaller  than  the  parotid.  It  is  situated  beneath  the 
mylohyoid  ridge,  and  occupies  the  anterior  part  of  the  sub- 
maxillary triangle,  extending  upAvard  to  occupy  the  submaxillary 
fossa  on  the  lower  border  of  the  maxilla.  Superficially,  it  is 
covered  by  the  skin  and  a  few  muscular  fibers  and  the  deep 
fascia.      The  facial  vein  and  branches  of  the  facial  nerve  pass 


GLANDS   OF  THE   MOUTH.  135 

over  its  superficial  surface.  Deeply,  it  is  in  relation  with  the 
mylohyoid  and  hyoglossus  muscles,  and  also  with  the  mylohyoid 
artery  and  nerve.  The  gland  receives  its  blood-  and  nerve- 
supply  from  the  arteries  and  nerves  which  penetrate  it.  This 
gland  is  separated  from  the  sublingual  gland  by  the  mylohyoid 
muscle,  and  weighs  about  two  drams. 

The  Submaxillary  Duct. — The  submaxillary  duct,  other- 
wise known  as  Wharton's  duct,  passes  forward  and  inward, 
opening  into  the  cavity  of  the  mouth  on  the  summit  of  a  small 
papilla  near  the  frenum  linguae.  The  duct,  which  is  nearly  two 
inches  in  length,  first  passes  through  the  adjacent  muscular  tis- 
sue, and  finally  beneath  the  oral  mucous  membrane  to  its  outlet. 
Like  the  parotid  duct,  it  is  lined  by  a  reflexion  of  the  oral 
mucous  membrane. 

The  Sublingual  Gland. — This  is  the  smallest  of  the  salivary 
glands,  and  is  also  named  from  its  location  beneath  the  tongue. 
Its  position  is  beneath  the  tip  of  the  tongue  and  the  mucous 
membrane  covering  this  part  of  the  floor  of  the  mouth.  It  rests 
in  the  submaxillary  fossa  of  the  maxilla,  meeting  with  its  fellow 
at  the  median  line,  and  extending  as  far  back  as  the  mylohyoid 
muscle,  which  separates  it  from  the  submaxillary  gland.  The 
gland  is  supplied  with  blood  from  the  sublingual  and  submental 
arteries,  and  with  nerves  from  the  gustatory.  The  weight  of 
this  gland  is  about  one  dram. 

The  Sublingual  Ducts. — This  gland  communicates  with  the 
mouth  by  one  large  duct — the  duct  of  Rivini — which  springs  from 
the  main  portion  of  the  gland,  and  by  a  number  of  smaller  ducts, 
eight  to  twenty  in  number,  which  open  on  the  floor  of  the  mouth. 
The  duct  of  Rivini  follows  the  submaxillary  duct,  and  opens 
with  it  at  the  same  papilla.  The  smaller  ducts  are  given  ofi" 
from  a  number  of  litde  lobes  which  cluster  about  the  fore  part 
of  the  sfland. 


CHAPTER   VIII. 

DESCRIPTION  OF  THE  UPPER  TEETH  IN  DETAIL.— CALCIFICA- 
TION, ERUPTION,  AND  AVERAGE  MEASUREMENTS.— THEIR  SUR- 
FACES, RIDGES,  FOSSAE,  GROOVES,  SULCI,  ETC. 

UPPER    CENTRAL    INCISOR. 


7th 

8th 
year 

Fig.  54. 

9th 
year 

Calcification  begins,  from  three  centers,  first  year  after  birth. 
Calcification  completed,  tenth  to  eleventh  year. 
Erupted,  seventh  to  eighth  year. 
Average  length  of  crown,  .39.* 

Average  length  of  root,  .49. 

Average  length  over  all,  .88. 

During  the  first  year  after  birth  this  tooth  begins  to  calcify, 
this  primitive  process  taking  place  along  the  future  cutting-edge 
of  the  tooth  in  three  distinct  lobes  or  plates,  which  afterward 
unite  and  form  three  eminences  or  tubercles,  the  lines  of  this 
union  being  indicated  upon  the  completed  crown  by  two  more 
or  less  defined  grooves — developmental  grooves.  By  the  end 
of  the  third  year  the  deposit  of  lime-salts  has  carried  the 
process  of  calcification  to  a  point  about  midway  between  the 
cutting-edge  and  the  cervical  line.  By  a  continuation  of  this 
formative    action  the   calcification   of  the  crown   is   completed 

"^The  measurements  given  are  in  fractions  of  an  inch,  and  are  taken  from  Black's  "  Dental 
Anatomy." 

136 


UPPER   CENTRAL   INCISOR.  137 

between  the  fifth  and  sixth  year.  At  the  beginning  of  the 
seventh  year  calcification  has  progressed  to  such  an  extent  that 
the  neck  of  the  tooth  and  base  of  the  root  are  fully  outlined. 
Between  the  seventh  and  eighth  year  the  cutting-edge  of  the 
tooth  begins  to  make  its  appearance  through  the  gum  at  a  point 
either  to  the  right  or  left  of  the  median  line,  and,  by  a  gradual 
separation  of  the  gum  tissue,  eruption  takes  place.  During 
the  following  year  about  one-eighth  of  an  inch  has  been  added 
to  the  length  of  the  root.  At  the  end  of  the  eighth  year  the 
root  has  become  calcified  to  about  one-half  of  its  completed 
length.  During  the  ninth  year,  owing  to  a  reduction  in  the 
diameter  of  the  root,  the  extent  of  growth  in  an  apical  direction 
has  almost  doubled  that  of  the  previous  year,  and  a  decided 
narrowing  of  the  free  root  margins  is  to  be  observed.  At  the 
eleventh  year  calcification  is  completed  in  the  outer  root  walls, 
and  the  apical  foramen  has  been  established  (Fig.  54). 

The  Crown  of  the  Upper  Central  Incisor  presents  for 
examination  four  surfaces — labial,  lingual,  mesial,  and  distal  ;  two 
angles — a  mesial  and  a  distal ;  and  a  cutting-edge.  The  general 
form  of  the  crown  is  that  of  a  double  Inclined  plane,  or  wedge- 
shape,  the  cutting-edge  representing  the  junction  of  the  two 
sides  of  the  incline,  one  of  which  looks  anteriorly  (labial)  and 
the  other  posteriorly  (lingual).  The  labial  side  of  the  incline  is 
convex,  while  the  lingual  is  concave  from  the  cutting-edge 
toward  the  root ;  but,  upon  reaching  its  upper  or  cervical  third, 
it  presents  a  slight  general  convexity.  The  base  of  the  wedge  Is 
directed  upward  and  partakes  of  the  contour  of  the  neck  of  the 
tooth. 

The  Labial  Surface  of  the  Crown  (Fig.  55).— In  general 
outline  this  surface  resembles  an  imperfect  quadrilateral.  The 
margins  of  the  surface  are  the  mesial,  the  distal,  the  cervical, 
and  the  incisive.  The  mesial  margin  begins  at  the  lower  border 
or  cutting-edge  and  passes  upward,  usually  with  a  slight  distal 
Inclination,  gradually  uniting  with  the  cervical  margin.  The 
distal  margin  begins  at  the  cutting-edge  and  passes  upward  with 
a  slight  mesial  inclination,  also  joining  the  cervical  margin.  Both 
of  these  margins  possess  more  or  less  general  convexity,  and, 
at   their  junction   with   the   cutting-edge,  form   the  mesial  and 


138  ANATOMY. 

distal  angles  of  the  crown.  The  cervical  margin  is  rounded  and 
gradually  passes  into  the  two  lateral  margins  just  described^' 
The  incisive  margin  is  marked  by  the  cutting-edge,  and  extends 
from  the  mesial  angle  on  one  side  to  the  distal  angle  on  the 
other.  These  four  margins,  which  assist  in  giving  to  the  tooth 
its  typal  form,  are  quite  variable.  This  difference  is  particularly 
marked  on  the  mesial  and  distal  margins,  where,  in  some  cases, 

there  is  a  decided  convergence 
in  the  direction  of  the  root,  form- 
ing what  is  commonly  termed 
the  bell-shaped  crown,  while  in 
others  the  same  margins  will  be 
nearly  parallel  with  each  other, 
making  the  width  of  the  crown 
almost  as  great  at  the  cervical 
margin  as  at  the  cutting-edge. 
Y\  ^H  '^^H  ]The    mesial    angle    is    usually 

pointed  and  square,  while  the 
distal  is  much  rounded.     This 
surface  of  the  crown  is  slightly 
Edge  Grooves  convex  irom  above  downward, 

^'"  "-l?bL\'  lr.lc\'.  ''""°^'         as  well  as  from  side  to  side,  and 

in  the  majority  of  instances  is  of 
greater  vertical  than  transverse  extent/^]  Beginning  at  the  inci- 
sive margin  are  two  slight  longitudinal  depressions  or  grooves — 
the  labial  grooves — which  are  resultant  from  the  developmental 
lobes  previously  composing  the  primitive  cutting-edge,  and,  for 
this  reason,  are  otherwise  known  as  developmental  grooves.  In 
many  instances  one  or  more  transverse  ridges  are  found  upon 
the  cervical  portion,  but  these  are  supplemental. 

The  Lingual  Surface  of  the  Crown  (Fig.  56). — This  surface 
has  its  borders  formed  by  three  marginal  ridges  and  the  cutting- 
■~f-  edge.  The  marginal  ridges  are  pronounced'elevations  of  enamel, 
and  surround  the  surface  upon  three  sides,  the  intervening  space 
in  many  instances  being  a  decided  concavity  or  fossa — the  lin- 
gual fossa.  ,  The  mesial  marginal  ridge  begins  at  the  mesial  angle 
of  the  crown,  passes  upward,  inward  and  backward,  following  the 
curvature  of  the  mesial  border. .  The  distal  mar ginal  ridgeh&gm?, 


UPPER   CENTRAL   INCISOR. 


at  the  distal  angle  of  the  crown  in  a  somewhat  less  pronounced 
form,  passes  upward,  backward,  and  inward,  following  the  cur- 
vature of  the  distal  surface.  Upon  reaching  the  cervical  portion 
of  the  crown  these  two  margins  unite  and  form  the  cervicoinar- 
ginal  ridge.  This  ridge  may  be  bold  and  prominent,  or  it  may 
be  but  slightly  developed.  Near  its  center  it  is  frequently  broken 
by  a  depression  or  pit — the  lingual  pit.  In  some  instances  this 
pit  is  deeply  penetrating;  in  others  it  assumes  the  form  of  a 
fissure,  and  may  completely  sever 
the  ridge.  This  border  is  some- 
times elevated  into  a  slightly  de- 
veloped tubercle  or  cusp — the 
cuspule.  When  this  is  present 
it  has  the  appearance  of  being 
produced  by  a  fold  of  enamel, 
and  encircling  it  is  a  well-marked 
fissure.  The  lingual  fossa  is 
usually  traversed  by  two  longi- 
tudinal grooves,  which  corres- 
pond to  the  developmental 
grooves  of  the  labial  surface. 
When  a  cuspule  is  present,  the 
fissure  which  surrounds  it  fre- 
quently passes  into  the  fossa,  or  the  fossa  may  be  partly  cov- 
ered by  the  cuspule  overhanging  its  cervical  portion.  When 
the  cervicolingual  fissure  exists,  it  is  not  unusual  for  it  to 
bifurcate  and  throw  a  branch  along  the  inner  border  of  each 
marginal  ridge,  or  it  may  penetrate  the  fossa  proper  and 
divide  it  into  two  parts.  The  lingual  surface  is  somewhat 
less  in  extent  than  the  labial  surface,  this  reduction  being 
principally  in  a  mediodistal  direction,  the  length  of  the  two 
surfaces  from  the  -cutting-edge  to  the  cervical  margin  being 
about  equal. 

The  Mesial  Surface  of  the  Crown  (Fig.  57). — The  outline 
of  this  surface  resembles  an  inverted  cone  or  triangle,  the  lines 
of  which  are  more  or  less  broken,  the  apex  of  the  cone  terminat- 
ing at  the  cutting-edge  and  the  base  directed  toward  the  root  of 
the  tooth.     The  base  of  the  cone  is  made  concave  by  the  enamel 


Fig.  56. — Upper  Central  Incisor, 
Lingual  Surface. 


margin  or  cervical  line.  The  margins  of  the  mesial  surface  are 
%  the  labial,  the  lingual,  and  the  cervical.  '  The  labial  margin  is 
convex  and  rounded  throughout  its  entire  extent,  from  the 
cutting-edge  to  the  cervical  line.  The  contour  of  this  margin 
varies  with  the  typal  form  of  the  crown,  in  some  presenting  a 
decided  and  well-marked  convexity,  in  others  being  but  slightly 
curved.  i^The  lingual  margin  is  concave  and  rounded,  but  the 
line  is  much  broken.    Beginning  at  the  cutting-edge,  it  is  decided 

and  square,  this  feature  usually 
including  the  lower  third.     As 
it  passes  upward  and  the  center 
is  approached,  the  line  is  more 
concave  and  rounded  in  a  mesio- 
lingual  direction,  this  latter  fea- 
ture increasing  upon  approach- 
\  ingthe  cervical  line.     The  cer- 
ige     vical  margin  is  that  formed  by 
the  cervical  line.     It  is  usually 
,,  .  ,  .    ,       well-defined,  beine  concave  or 

Mesial  Angle  o 

V-shaped,  with  the  point  of  the 
V  more  or  less  rounded,  and 

Fig.  57. — Upper  Central  Incisor,  .  ,    .       ^  ,  .      . 

Mesial  Surface.  With  its  tree  ends  pointing  one 

in  a  labial  and  one  in  a  lingual 
direction,  the  former  being  a  trifle  longer  than  the  latter.^  The 
surface  between  the  borders  presents  a  slight  general  convexity, 
but  with  an  inclination  to  flatness  near  the  cervical  portion, 
which  is  occasionally  developed  into  a  slight  concavity.  What- 
ever deviations  may  be  present  in  the  borders  of  this  surface, 
from  those  assumed  in  the  description  just  given,  their  union  at 
the  cutting-edge  will  always  be  in  a  direct  line  with  the  long 
axis  of  the  tooth. 

The  Distal  Surface  of  the  Crown  (Fig.  58). — In  a  general 
way,  this  surface  resembles  the  mesial  surface  just  described. 
There  are,  however,  one  or  two  minor  points  of  distinction  :  the 
borders  are  all  more  rounded,  the  labial  border  presenting  a 
greater  convexity,  and  the  lingual  a  more  perfectly  formed  con- 
cavity. The  surface  is  quite  full  in  the  center,  from  which  it  slopes 
away  in  all  directions,  thus  producing  a  decided  general  convexity. 


UPPER   CENTRAL   INCISOR. 


141 


Ridge 


Lingual 
Ridge 


Fig.  58. — Upper  Central  Incisor, 
Distal  Surface. 


The  cervical  margin  of  the  surface  is  almost  identical  with  the 
cervical  margin  of  the  mesial  surface.  The  distance  in  a  direct 
line  from  the  cervical  border  to 
the  cutting-edge  is  a  trifle  less 
than  the  corresponding  meas- 
urement on  the  mesial  surface. 
The  distal  angle  is  equally  con- 
stant in  its  position,  and,  being 
connected  with  the  mesial  an- 
gle in  a  direct  line  by  the  cut- 
ting-edge, finds  this  latter  cor- 
onal margin  always  in  the  labio- 
lingual  center  of  the  crown. 

The  Ctitting-edge  of  the 
Central  Incisor. — The  cutting 
or  incisive  edge  receives  its 
name  from  its  function,  that  of 
cutting  or  incising  the  food.  It  is  formed  by  the  junction  of  the 
labial  and  lingual  surfaces  of  the  crown,  and  extends  almost  in  a 
direct  line  from  the  mesial  to  the  distal 
surface ;  at  its  union  with  the  mesial  sur- 
face it  assists  in  forming  the  mesial  angle 
of  the  crown,  and  serves  the  same  purpose 
by  its  union  with  the  distal  surface.  In  the 
majority  of  instances  it  is  an  unbroken 
line.  In  passing  from  the  mesial  to  the 
distal  angle  it  converges  slightly  in  the 
direction  of  the  root,  thus  making  the 
crown  a  trifle  shorter  on  the  mesial  than 
on  the  distal  side.  In  the  recently  erupted 
tooth  (Fig.  59)  the  line  is  broken  by  the 
developmental  grooves ;  these  usually 
disappear  by  wear,  but  occasionally  traces 
of  their  existence  remain,  and  thus  per- 
manently break  the  positive  line  that 
would  otherwise  be  present.  As  the  cut- 
ting-edge approaches  the  distal  angle  of  the  crown  it  is  inclined 
to  slope  away,  producing  a  less  positive  angle  than  the  corre- 


FiG.  59. — A  Young  Upper 
Central  Incisor,  Labial 
Surface,  showing  Develop- 
mental Grooves — a. 


spending  mesial  angle.  In  some  instances  the  cutting-edge  is 
quite  thin  and  inclined  to  sharpness,  in  others  it  is  blunt  and 
dull,  the  former  condition  being  present  when  there  is  a  decided 
overbite  in  occlusion,  the  latter  occurring  when  this  feature  is 
less  pronounced.  The  cutting-edge  is  frequently  referred  to 
as  the  occlusal  surface,  this  term  being  employed  to  make  the 
description  more  uniform  with  the  bicuspids  and  molars,  and 
for  this  reason  is  permissible  ;  but  it  is  only  in  rare  instances 
that  the  surfaces  occlude  directly  with  the  opposing  teeth,  the 
condition  being  most  frequent  in  teeth  of  the  lymphatic  type, 
and  in  cases  of  malocclusion. 

The  Cervical  Margin. — This  margin,  which  is  distinctly 
outlined  by  the  free  extremity  of  the  enamel  covering  of  the 
crown,  also  marks  the  extent  of  the  membrane  covering  the 
root  of  the  tooth.  The  margins  formed  by  this  line  are  those 
of  a  double  concavity  and  a  double  convexity.  On  the  labial 
and  lingual  portions  it  is  concave  rootward,  while  on  the 
mesial  and  distal  sides  it  is  convex  in  this  direction.  If  a 
line  be  drawn  around  the  tooth  at  the  extreme  upper  point 
of  the  enamel  covering,  it  will  be  found  to  touch  only  the  labial 
and  lingual  prolongations,  while  a  space  will  exist  between  the 
line  thus  drawn  and  the  cervical  margins  of  the  proximate  sur- 
faces. The  character  of  the  cervical  curvature  varies  with  the 
type  of  the  tooth,  being  more  or  less  pronounced  as  the  case 
may  be.  In  a  typical  central  incisor  the  cervical  line  of  the  labial 
surface  will  usually  form  the  segment  of  a  larger  circle  than  that 
of  the  lingual,  and,  while  the  mesial  convexity  may  be  gracefully 
curved,  the  distal  may  incline  to  angularity. 

The  Neck  of  the  Tooth. — The  neck  of  this  tooth  partakes 
of  a  form  between  that  of  the  crown  and  root  which  it  joins.  It 
is  principally  formed  by  a  sudden  sloping  of  the  enamel  margin 
to  meet  the  root.  It  is  broader  on  the  labial  than  on  the  lingual 
surface,  and  is  somewhat  flattened  laterally,  with  an  occasional 
depression  or  concavity  on  its  mesial  portion.  The  neck  of  this 
tooth  is  seldom  a  decided  anatomic  feature,  being  less  pro- 
nounced than  upon  any  other  tooth.  In  the  bicuspids  and 
molars  both  the  crowns  and  the  roots  assist  in  forming  the  neck 


UPPER   CENTRAL   INCISOR.  I43 

by  a  constriction  of  their  adjacent  parts,  while  in  this  tooth  the 
crown  alone  is  instrumental  in  this  direction. 

The  Root  of  the  Upper  Central  Incisor. — The  root  of 
this  tooth  is  conic  in  form,  its  base  directed  downward,  its  apex 
upward.  ''"Viewed  in  transverse  sections  its  outline  is  that  of  a 
rounded  triangle,  one  side  of  which  faces  in  a  labial,  one  in  a 
mesiolingual,  and  one  in  a  distolingual  direcdon.  The  labial 
side  is  the  most  flattened,  while  the  two  remaining  sides  are  of 
equal  length  and  oval  in  form.  This  triangular  oudine  usually 
condnues  throughout  the  entire  length  of  the  root,  but  in  some 
instances,  near  the  apical  end,  may  have  a  decided  or  slight  distal 
curve,  included  in  which  will  be  a  more  circular  form.  ^  The  taper 
of  the  root  from  the  base  to  the  apex  is  very  gradual  upon  the 
labial  and  lingual  surfaces,  until  the  apical  third  is  reached,  when 
the  two  sides  converge  more  rapidly.  The  mesial  and  distal 
surfaces  are  somewhat  flattened  and  taper  very  gradually  from 
the  base  to  the  apex.  In  a  majority  of  instances  the  root  is 
much  longer  than  the  crown,  but  in  rare  cases  its  length  is 
barely  equal  to,  or  less  than,  that  of  the  crown. 

Bilious  Type. — The  crown  of  the  upper  central  incisor  in 
this  type  is  of  greater  longitudinal  than  transverse  extent ;  large 
in  size,  abounding  in  angles  rather  than  curves.  It  possesses 
neither  brilliancy  nor  transparency  of  surface,  but  is  slightly  in- 
clined to  translucency.  The  labial  surface  is  flat,  with  more  or 
less  decided  transverse  ridges  in  the  cervical  portion.  The 
labial  grooves  are  generally  present  in  the  form  of  well-defined 
depressions.  On  account  of  the  angular  nature  of  the  tooth, 
this  surface  approaches  closely  to  the  quadrilateral  form.  The 
mesial  and  distal  surfaces  are  flat,  with  their  margins  bold  and 
well  defined.  The  lingual  surface  also  shows  the  angular  nature 
of  the  crown  in  having  its  marginal  ridges  squarely  set  and  its 
developmental  grooves  definitely  outlined.  The  cutting-edge 
is  rather  thin,  square,  and  sharp,  the  line  frequently  being  im- 
perfectly formed.  The  mesial  and  distal  angles  are  both  well 
produced,  and  the  cervical  margin,  in  keeping  with  the  rest  of 
the  parts,  is  Inclined  to  angularity. 

Nervous  Type. — The  central  incisor  common  to  this  tem- 
perament is  delicate  and  graceful  in  outline.     The  crown  is  of 


144  ANATOMY. 

medium  size,  witli  tlie  length  predominating  over  breadth.  The 
enamel  is  inclined  to  transparency,  and  is  of  a  blue  or  bluish- 
gray  color,  presenting  much  brilliancy.  The  labial  surface  is 
fairly  well  rounded,  and  the  labial  grooves  are  present  as  slightly 
rounded  depressions,  which  frequently  extend  well  toward  the 
cervical  margin,  where  they  gradually  disappear.  In  general 
outline  this  surface  partakes  of  the  triangular  form,  the  crown 
of  the  tooth  being  broad  at  the  cutting-edge  and  much  con- 
stricted at  the  neck.  The  mesial  and  distal  surfaces  show  a 
convexity  in  every  direction,  and  the  nature  of  the  occlusion  is 
manifest  from  the  decided  wedge-shape  appearance  of  the  crown 
providing  for  a  long  overbite.  Upon  the  lingual  surface  but 
little  in  the  way  of  detail  is  to  be  observed,  the  entire  surface 
from  the  cutting-edge  to  the  cervical  ridge  being  smooth  and 
concave.  The  cuspule  previously  referred  to  is  occasionally 
present  in  this  type,  breaking  the  general  smoothness  of  the 
surface  with  its  prominence.  The  marginal  ridges  are  poorly 
defined  ;  the  cutting-edge  is  a  sharp,  unbroken  line  ;  the  mesial 
and  distal  angles  are  present  in  the  form  of  long,  graceful 
curves,  rather  than  definite  angles,  this  being  particularly  true 
of  the  distal  angle.  The  cervical  line  is  decidedly  curved,  the 
labial  and  lingual  portions  being  deeply  concave  rootward,  while 
the  mesial  and  distal  are  decidedly  convex. 

Sanguineous  Type. — The  crown  of  the  central  incisor  is 
usually  above  the  average  in  size,  but  is  well  proportioned, 
abounding  in  curves  and  rounded  outlines.  The  enamel  is 
inclined  to  translucency,  particularly  near  the  cutting-edge. 
The  labial  surface  is  smooth  and  rounded ;  the  depressions 
formed  by  the  labial  grooves  are  slightly  observable,  and  ex- 
tend but  a  short  distance  from  the  cutting-edge.  The  surface 
is  somewhat  greater  in  longitudinal  than  in  transverse  extent, 
and  approaches  much  nearer  to  a  circular  form  than  the  corre- 
sponding tooth  of  other  types.  The  mesial  and  distal  surfaces 
are  well  rounded,  making  the  point  of  contact  with  approxi- 
mating teeth  near  the  center  of  the  surface.  The  lingual  sur- 
face abounds  in  heavy  rounded  lines  ;  the  marginal  and  cervical 
ridges  are  particularly  prominent,  diminishing  the  extent  of  the 
lingual  fossa.     A  cuspule  is  frequently  present  in  the  form  of  a 


UPPER    CENTRAL   INCISOR.  145 

well-rounded  prominence.  The  cutting-edge  is  of  moderate  thick- 
ness and  slopes  away  from  the  center  in  either  direction  to  assist 
in  forming  the  rounded  mesial  and  distal  angles.  The  cervical 
curvature  on  the  labial  and  lingual  surfaces  is  an  unbroken  semi- 
circle, while  that  of  the  mesial  and  distal  surfaces  is  less  uniform. 

Lymphatic  Type. — In  the  central  incisor  of  this  typal  form 
the  crown  is  large,  but  not  shapely,  and  the  breadth  is  equal 
to,  or  exceeds,  the  length.  The  enamel  coloring  is  muddy  or 
brownish-yellow,  and  the  surface  is  lacking  in  brilliancy.  The 
labial  surface  is  flat  and  smooth,  with  a  faint  sign  of  the  labial 
grooves.  The  general  outline  of  this  surface  is  that  of  a  circular 
cone,  with  the  cutting-edge  for  the  base,  and  the  apex  formed 
on  the  cervical  margin.  The  mesial  and  distal  aspects  present 
a  striking  contrast  to  the  types  previously  described,  by  having 
a  labiolingual  diameter  greater  than  that  represented  between 
the  cervical  line  and  the  cutting-edge.  These  two  surfaces  are 
convex  in  a  labiolingual  direction  only,  making  the  point  of 
contact  with  approximating  teeth  an  extended  surface  rather 
than  a  single  point. 

The  lingual  surface  is  heavy  and  bulky,  frequently  to  such  a 
degree  as  to  produce  a  general  convexity  rather  than  a  concav- 
ity, as  found  in  most  typal  forms.  This  surface  is  frequently 
broken  by  one  or  more  longitudinal  grooves,  but  is  seldom 
crossed  by  transverse  lines  of  any  kind.  The  cutting-edge  is 
barely  deserving  of  the  name.  Although  formed  by  the  free 
borders  of  the  labial  and  lingual  surfaces,  these  two  planes  are 
so  far  separated  at  their  incisive  margins  that  the  space  between 
them,  instead  of  being  an  edge,  becomes  a  more  or  less  broad- 
ened surface,  and  one  upon  which  the  lower  incisors  frequently 
occlude.  The  line  thus  formed  is  straight  and  direct  from  the 
mesial  to  the  distal  angle  of  the  crown,  both  of  which  are  well 
produced.  The  cervical  curvature  is  represented  by  the  segment 
of  a  much  larger  circle  than  that  found  upon  teeth  of  other 
types,  and  the  neck  of  the  tooth  is  heavy  and  bulky,  showing 
but  little  constriction  at  this  point. 


146 


UPPER  LATERAL  INCISOR. 


4th  5th 

year  year 


Fig.  60. 


Calcification  begins,  from  three  centers,  first  year  after  birth. 
Calcification  completed,  tenth  to  eleventh  year. 
Erupted,  seventh  to  eighth  year. 
Average  length  of  crown,  .34. 

Average  length  of  root,  .51. 

Average  length  over  all,  .85. 

Like  the  central  incisor,  calcification  in  this  tooth  begins  during 
the  first  year  after  birth,  the  process  taking  place  in  the  same 
manner,  firom  three  centers,  along  the  future  cutting-edge,  and 
gradually  extending  in  the  direction  of  the  root.  By  the  expi- 
ration of  the  third  year  the  cutting-edge  and  the  angles  of 
the  crown  are  fully  formed ;  the  fourth  year  finds  the  crown 
calcified  to  nearly  one-half  its  completed  length ;  by  the  fifth 
year  the  cervical  ridge  is  reached ;  while  the  sixth  year  usually 
completes  the  process  of  coronal  calcification.  At  the  close  of 
the  seventh  year  the  base  of  the  root  is  fully  outlined,  and 
during  the  following  year  about  one-eighth  of  an  inch  is  added 
to  its  length,  and  still  greater  progress  is  made  during  the  ninth 
year,  by  which  time  fully  three-fourths  of  the  root  length  has 
become  calcified.  During  the  tenth  year  the  apical  end  of  the 
root  begins  to  form  by  a  sudden  doubling-over  of  the  free  cal- 
cifying margins,  and  by  the  eleventh  year  the  apical  foramen  is 
established  (Fig.  60).  By  the  above  description  it  will  be 
observed  that  at  the  time  of  eruption  the  root  of  this  tooth  is 
only  calcified  to  about  one-half  of  its  completed  length,  and  the 
same  may   be   said   of  the  central  incisor ;    but   so  much  time 


UPPER   LATERAL   INCISOR. 


elapses  between  the  beginning  of  the  eruptive  stage  and  the 
period  at  which  this  phenomenon  is  completed  that  the  apical 
foramen  is  usually  established  by  the  time  the  tooth  assumes  its 
permanent  position  in  the  jaw. 

The  crown  of  the  upper  lateral  incisor,  like  that  of 
the  upper  central  incisor,  presents  for  examination  four  sur- 
faces,— labial,  lingual,  mesial,  and  distal, — a  cervical  margin,  a 
cutting-edge,  and  a  mesial  and  distal  angle.  The  general  con- 
tour of  the  crown  closely  resembles  that  of  the  upper  central 
incisor,  except  that  it  measures  about  one-third  less  from  mesial 
to  distal,  and  is  a  trifle  shorter  from  the  cutting-edge  to  the  cer- 
vical line.  As  in  the  central  incisor,  the  labial  and  distal  surfaces 
form  a  double  incline  plane,  and 
unite  below  to  form  the  cutting- 
edge.  The  labial  side  of  the  in- 
cline is  convex,  while  the  lingual 
is  concave,  but  seldom  so  marked 
as  that  found  upon  the  central 
incisor.  The  base  of  the  wedge, 
or  double  incline,  formed  by  the 
cervical  margin,  is  correspond- 
ingly smaller  than  that  of  the 
crown  of  the  central  incisor. 

The  Labial  Surface  of  the 
Crown  (Fig.  61). — This  surface 
of  the  crown  of  the  upper  lat- 
eral incisor  is  more  irregular  in 
outline  than  the  corresponding 
surface  of  the  central  incisor.   The 

margins  of  the  surface  are  the  mesial,  distal,  cervical,  and  incisive. 
The  mesial  margin  begins  at  the  mesial  angle  and  passes  upward 
with  a  decided  distal  inclination  to  meet  the  cervical  margin. 
The  distal  margin  is  shorter  and  decidedly  more  convex  than 
the  mesial  margin,  this  variation  in  outline  being  still  more 
marked  when  compared  with  the  corresponding  margin  of  the 
central  incisor.  At  the  cutting-edge  these  two  margins  assist 
in  forming  the  mesial  and  distal  angles  of  the  crown,  and  by 
their  continuation  and  union  above  form  the  cervical  margin. 


labial 
Ridge 


Labial  Grooves 

Fig.  61. — Upper  Lateral  Incisor, 
Labial  Surface. 


I4S 


ANATOMY. 


The  incisive  margin  is  formed  by  the  cutting-edge.  Like  the 
central  incisor,  the  four  margins  of  this  surface  vary  greatly  in 
the  different  types ;  this  is  particularly  true  of  the  two  lateral 
margins,  which  at  times  are  found  to  be  in  the  form  of  a  direct 
line,  or  even  slightly  concave,  while  in  others  they  are  both 
decidedly  convex.  This  surface  of  the  crown  shows  a  greater 
general  convexity  than  the  labial  surface  of  the  central  incisor, 
the  cervical  portion  presenting  a  curve  much  more  decided  than 
that  near  the  cutting-edge.  The  labial  grooves  are  in  all 
respects  similar  to  those  described  in  connection  with  the  central 
incisor,  and  extend  from  the  cutting-edge  toward  the  center  of 

the  surface,  where  they  grad- 
ually disappear.  Transverse 
ridges  are  occasionally  found 
near  the  cervical  portion  of  the 
surface. 

The  Lingual  Surface  of 
the  Crown  (Fig.  62). — This 
surface  of  the  upper  lateral  in- 
cisor is  subject  to  much  varia- 
tion in  form,  but  presents  the 
same  points  for  examination  as 
the  corresponding  surface  of  the 
central  incisor.  These  consist 
of  the  marginal  ridges,  which 
are  usually  more  pronounced 
than  those  of  the  central,  mak- 
ing the  concavity  or  fossa  between  them  small  and  deep. 
In  some  instances  the  surface  will  be  smooth  and  flat,  with 
an  entire  absence  of  ridges  or  fossae.  The  distal  marginal 
ridge  is  shorter  and  more  bowed  than  the  mesial,  and  the 
cervical  ridge  is  well  marked  and  proportionately  broader  and 
stronger  than  in  the  central  incisor.  In  some  instances  the 
marginal  ridges  are  but  slighdy  developed,  with  their  cer- 
vical ends  broadened  and  separated  by  a  deep  fissure,  giving 
the  appearance  of  a  terminal  fold  in  the  enamel.  The  cervical 
ridge  is  frequently  broken  by  a  cuspule,  which  is  usually 
more  pronounced  than  when  found  upon  the  central   incisor. 


ginal  Ridge 


Fig.  62. — Upper  Lateral  Incisor, 
Lingual  Surface. 


UPPER   LATERAL   INCISOR. 


Fig.  63.  —  Upper 
Lateral  Incisor, 
Mesial  Surface. 


The  lingjial  fossa  may  be  present  as  a  smooth,  unbroken  con- 
cavity, or  it  may  be  subdivided  by  a  longitudinal  ridge,  which 
often  exists  to  such  an  extent  as  to  force  the  remaining  portions 
of  the  fossa  well  against  the  marginal  ridges, 
where  they  will  be  observed  as  slight  depres- 
sions rather  than  marked  concavities. 

The  Mesial  Surface  of  the  Crown  (Fig. 
63). — Viewing  the  crown  from  this  aspect, 
the  outline  is  that  of  an  inverted  cone  or 
triangle.  The  lingual  margin  of  the  surface 
is  well  defined,  and  the  angle  formed  by  the 
union  of  this  surface  with  the  lingual  surface 
is  moderately  acute.  The  labial  margiti  is 
well  rounded,  and  passes  into  the  labial  sur- 
face without  a  decided  line  of  demarcation. 
The  surface  on  its  upper  or  cervical  third  is 
usually  flattened  and  occasionally  concave. 
At  the  center,  and  condnuing  toward  the 
cutting-edge,  it  is  decidedly  convex  in  every 
direction,  thus  producing  a  prominent  point  of  contact  with  the 
distal  surface  of  the  central  incisor. 

The  Distal  Surface  of  the  Crown  (Fig.  64). — This  surface 
also  shows  the  characterisdc  wedge-shape  ot 
the  crown,  and  is  principally  different  from  the 
mesial  surface  in  being  convex  throughout. 
Near  the  center  it  is  well  rounded  and  full,  pro- 
viding a  point  of  contact  for  the  mesial  surface 
of  the  cuspid.  The  lingual  margin,  while  being 
more  decidedly  outlined  than  the  labial,  is  much 
more  rounded  than  the  lingual  margin  of  the 
mesial  surface.  From  the  most  prominent  point 
near  its  center  the  surface  slopes  away  in  every 
direction,  the  convexity  being  most  marked 
near  the  cutting-edge. 

The  Cutting-edge  of  the  Lateral  Incisor. — In 
the  young  tooth  the  cutting-edge  presents  the 
three  litde  tubercles  common  to  all  incisors,  the 
grooves  which  divide  them  passing  up  over  the  labial  and  lin- 


FiG.  64. — Upper 
Lateral  Incisor, 
Distal  Surface. 


glial  surfaces  and  forming  the  labial  and  lingual  grooves.  These 
soon  disappear  by  wear,  usually  leaving  the  cutting-edge  in  the 
form  of  a  direct  line  and  connecting  the  two  angles  of  the  crown. 
Like  the  central  incisor,  this  margin  of  the  crown  may  be  thin 
and  sharp,  or  it  may  be  thick  and  dull. 

The  Cervical  Margin, — The  line  of  demarcation  between  the 
crown  and  the  root  of  the  tooth  resembles  so  closely  that  de- 
scribed in  connection  with  the  central  incisor  that  it  will  only  be 
necessary  to  mention  one  or  two  characteristic  differences. 
The  lingual  side  of  the  line  presents  a  much  smaller  curve  pro- 
portionately, and  usually  extends  a  little  higher  in  the  direction 
of  the  root  than  that  represented  upon  the  labial  portion.  The 
mesial  and  distal  portions  of  the  line  dip  well  down,  decreasing 
the  length  of  the  crown  on  these  surfaces  ;  the  margin  on  the 
former  surface  is  usually  angular  and  V-shaped,  while  on  the 
latter  it  is  circular  in  form. 

The  Angles  of  the  Crozuii. — The  angles  of  the  crown  are  the 
mesial  and  the  distal,  and  are  formed  in  the  same  manner  as  the 
same  angles  of  the  central  incisor.  The  mesial  angle  is  generally 
well  produced,  in  most  instances  being  slightly  acute  ;  but  when 
the  cutting-edge  is  thin  and  frail,  the  angle  is  frequently  much 
obliterated  by  wear.  That  portion  of  the  crown  of  the  upper 
lateral  incisor  which  is  usually  referred  to  as  the  distal  angle  is 
scarcely  worthy  of  the  name.  It  is  usually  present  as  a  long 
curve,  which  begins  near  the  center  of  the  cutting-edge  and  ex- 
tends well  up  on  the  distal  surface.  This  characteristic  outline  is 
sometimes  so  pronounced  as  to  completely  destroy  the  cutting- 
edge,  the  distal  surface  being  carried  forward  by  a  long  curve 
ending  in  the  mesial  angle. 

The  Neck  of  the  Tooth. — In  this  tooth  the  neck  is  usually 
marked  by  a  constriction  much  more  pronounced  than  that 
found  in  the  central  incisor.  On  the  labial  and  lingual  sur- 
faces it  is  principally  formed  by  a  sudden  sloping  of  the  enamel 
surface  rootward,  but  on  the  two  lateral  surfaces  it  is  formed 
by  a  flattening  or  slight  concavity  of  both  the  crown  and  the 
root. 

The  Root  of  the  Upper  Lateral  Incisor. — The  root 
of  this  tooth  is  conic  in  form,  and  is  much  more  flattened  from 


UPPER    LATERAL   INCISOR.  151 

mesial  to  distal  than  the  root  of  the  central  incisor.  At  its 
junction  with  the  crown  it  is  circular  in  form,  the  labial  portion 
forming  the  segment  of  a  larger  circle  than  the  lingual,  this  feature 
being  observed  throughout  its  entire  length.  The  flattening  of 
the  mesial  and  distal  sides  begins  immediately  above  the  neck, 
and  gradually  increases  as  the  center  of  the  root-length  is  ap- 
proached, where  it  often  develops  into  a  slight  longitudinal  de- 
pression. As  the  apical  end  of  the  root  is  reached,  this  longi- 
tudinal depression  gradually  disappears,  and  the  root  again 
becomes  circular  in  form.  The  thickness  of  the  root  is  about 
one-third  greater  from  labial  to  lingual  than  from  mesial  to 
distal,  and,  while  it  is  generally  classed  as  a  straight  root,  it  is 
frequently  provided  with  a  pronounced  distal  curve  near  the 
apical  extremity.  In  some  instances  it  is  found  with  a  double 
mesiodistal  curve. 

Bilious  Type. — In  this  type  the  lateral  incisor  is  frequently 
poorly  developed,  the  cutting-edge  and  distal  angle  are  want- 
ing, the  crown  being  in  the  form  of  a  single  conic  cusp,  the 
distal  surface  meeting  the  mesial  at  a  point  near  the  mesial 
angle.  This  form  of  crown  might  be  classed  as  one  of  malfor- 
mation, but  the  fact  that  it  most  frequently  occurs  in  this  tem- 
perament would  appear  to  indicate  a  normal  condition.  When 
the  crown  takes  the  form  common  to  incisors,  it  is  of  greater 
longitudinal  than  transverse  extent,  the  angles  are  well  pro- 
duced, and  the  mesial  and  distal  surfaces  are  flat  and  almost  par- 
allel with  each  other.  The  labial  surface  is  flat  and  is  frequently 
broken  by  transverse  ridges  near  the  cervical  portion.  The 
lingual  surface  presents  well-marked  outlines  and  margins,  a  cus- 
pule  is  seldom  present,  and  the  lingual  fossa  is  well  marked,  but 
not  deep.  The  cutting-edge  is  thin  and  sharp,  to  provide  for 
the  overbite,  which  is  rather  long.  The  cervical  border  is  square 
and  angular. 

Nervous  Type. — In  this  typal  form  the  neck  of  the  tooth 
is  a  pronounced  feature.  The  crown  is  long  and  narrow,  the 
constriction  forming  the  neck  beginning  well  down  on  the  crown 
and  extending  over  the  cervical  line  to  the  surface  of  the  root. 
The  labial  surface  is  convex  in  every  direction  and  the  labial 
grooves  fairly  well  defined.     The  mesial  surface  is  convex  near 


152  ANATOMY. 

the  center  and  cutting-edge,  but  often  shows  a  sHght  concavity 
on  its  cervical  portion.  The  distal  surface  is  rounded  and  smooth. 
The  lingual  surface  presents  a  general  concavity,  a  cuspule 
being  more  frequently  present  than  in  other  types.  The  lingual 
fossa  is  deep,  and  often  extends  beneath  the  cervicomarginal 
ridge  in  the  form  of  a  circular  fissure.  The  cutting-edge  is 
thin  and  sharp,  providing  for  a  long  overbite  ;  the  mesial  angle 
is  pointed  and  well  formed,  while  the  distal  is  usually  much 
rounded.  The  cervical  line  is  well  arched,  forming  the  segment 
of  a  much  smaller  circle  than  that  seen  on  the  same  tooth  of 
other  temperaments. 

Sanguineous  Type. — The  crown  is  well  proportioned,  with 
the  length  slighdy  predominating  over  breadth,  all  the  surfaces 
being  more  or  less  rounded  and  smooth,  showing  the  crown 
to  be  made  up  of  curves  rather  than  angles.  The  labial  surface 
presents  a  graceful  convexity  throughout ;  the  mesial  and  distal 
surfaces  are  both  convex,  with  their  margins  poorly  defined. 
The  lingual  surface  shows  the  rounded  nature  of  the  crown  in 
having  its  fossa  and  marginal  ridges  oval  and  blending  one  into 
the  other.  The  cutting-edge  is  moderately  heavy  and  dull,  in 
keeping  with  the  overbite,  which  is  short.  The  cervical  line  is 
made  up  of  curves  rather  than  angles. 

Lymphatic  Type. — In  this  type  the  crown  is  generally  of 
greater  transverse  than  longitudinal  extent.  The  neck  is  poorly 
produced,  the  crown  and  root  uniting  without  any  marked  con- 
striction of  the  parts.  The  labial  surface  is  much  flattened  from 
mesial  to  distal,  and  but  slightly  convex  in  the  direction  of  the 
long  axis  of  the  tooth.  The  mesial  and  distal  surfaces  are  but 
little  rounded  and  are  nearly  parallel  with  each  other,  so  that  the 
contact  with  adjoining  teeth  becomes  an  extent  of  surface  rather 
than  a  single  point.  The  lingual  surface  is  convex  above,  but 
as  the  cutting-edge  is  approached  it  becomes  flat,  but  seldom 
concave.  The  marginal  ridges  are  not  well  shown  and  the 
lingual  fossa  is  but  a  slight  depression.  The  angles  of  the 
crown  are  well  produced  and  the  cutting-edge  thick  and  blunt, 
this  marginal  surface  frequently  occluding  directly  upon  the 
opposing  lower  teeth.  The  curvature  of  the  cervical  line  is 
that  of  a  long  circle. 


UPPER   CUSPID. 


153 


UPPER    CUSPID. 


^    IPf    f 


Fig.  65. 

Calcification  Begins,  from  Three  Centers,  Third  Year  after  Birth. 
Calcification  Completed,  Twelfth  to  Thirteenth  Year. 
Erupted,  Twelfth  to  Thirteenth  Year. 
Average  Length  of  Crown,  .37. 

Average  Length  of  Root,  .68. 

Average  Length  over  All,  1.05. 

About  the  third  year  after  birth  calcification  begins  in  the 
central  lobe,  which  is  gradually  extended  laterally,  until,  at  the 
fourth  year  it  is  met  by  the  two  lateral  lobes,  which  are  some- 
what later  in  beginning,  and  by  the  fifth  year  the  three  are 
united,  the  former  eventually  establishing  the  single  cusp  of  the 
tooth  and  the  latter  two  the  mesial  and  distal  angles.  About 
the  sixth  year  two-thirds  of  the  crown  is  formed,  and  by  the 
seventh  year  the  constriction  which  marks  the  beginning  of  the 
neck  of  the  tooth  commences  to  make  its  appearance.  Between 
the  seventh  and  eighth  year  calcification  in  the  crown  is  com- 
pleted and  the  cervical  line  established,  during  the  following  year 
nearly  one-quarter  of  an  inch  is  added  to  the  length  of  the  root, 
and  by  the  beginning  of  the  tenth  year  the  root  is  formed  for  fully 
two-thirds  of  its  entire  length.  Between  the  twelfth  and  thirteenth 
years  or  at  the  time  of  eruption,  calcification  is  completed  in  the 
root  and  the  apical  foramen  established  (Fig.  65).  In  this  latter 
particular  the  cuspid  tooth  differs  from  most  of  the  others  in 


154  ANATOMY. 

being  completely  calcified  previous  to,  or  about  the  time  of, 
its  eruption,  the  eruptive  process  in  this  tooth,  therefore, 
differs  from  that  of  the  other  teeth,  being  fully  calcified  about 
the  time  it  makes  its  appearance  through  the  gum  tissue. 
To  reach  its  final  position  in  the  arch  the  tooth  moves  bodily 
downward,  the  bone  filling  in  behind  ;  while  in  the  incisors,  bi- 
cuspids, and  molars  the  free  calcifying  root-extremities  remain 
nearly  stationary,  the  crowns  being  forced  downward  as  the  lime 
salts  are  deposited. 

The  crown  of  the  upper  cuspid  presents  for  examination 
four  surfaces, — labial,  lingual,  mesial,  and  distal, — two  margins, — 
the  cervical  margin  and  the  cutting-edge, — and  a  mesial  and  a 
distal  angle.  In  general  outline  it  is  of  the  simplest  form,  re- 
sembling the  primitive  cone-shaped  teeth  of  many  fishes.  When 
viewed  by  looking  directly  upon  the  mesial  or  distal  surface,  the 
wedge-shape  common  to  the  incisors  is  observed.  The  base  of 
the  double  incline  is,  however,  much  broader  proportionately 
than  the  corresponding  measurement  of  the  incisors.  Looking 
at  the  crown  from  a  labial  or  lingual  direction,  its  function, 
as  both  a  penetrating  and  incising  organ,  may  be  observed 
in  the  single  cusp  from  which  it  derives  its  name.  The  cusp, 
which  is  formed  at  the  expense  of  the  cutting-edge,  divides  this 
latter  margin  into  two  distinct  portions — the  mesial  cutting-edge 
and  the  distal  cuttitig-edge . 

The  Labial  Surface  of  the  Crown  (Fig.  66). — The  contour 
of  this  surface  is  that  of  a  broken  circle  more  or  less  perfectly 
drawn.  It  is  bounded  by  five  margins — mesial,  distal,  cervical, 
mesial-incisive,  and  distal-incisive.  The  mesial  margin  is  rounded 
from  labial  to  mesial,  and  slightly  convex  from  the  cutting-edge 
to  the  cervical  line.  The  distal  margin  is  also  rounded  from 
labial  to  distal,  presents  a  greater  convexity,  and  is  somewhat 
shorter  from  the  cutdng-edge  to  the  cervical  line  than  the  mesial 
margin.  By  a  continuation  and  final  union  of  these  two  lateral 
margins  the  cervical  margin  of  the  surface  is  formed,  while  by 
their  union  with  the  cutting-edges  the  mesial  and  distal  angles 
of  the  crown  are  established.  The  mesial-incisive  margin  is  usu- 
ally slightly  concave  near  its  center,  although  in  some  instances 
it  is  convex.     The  distal  incisive  margin  responds  to  the  same 


UPPER   CUSPID. 


155 


description,  although  the  concavity,  when  present,  is  nearest  the 
point  of  the  cusp.  t^From  the  summit  of  the  cusp  these  two 
margins  slope  away  to  join  the  mesial  and  distal  angles,  the_ 
distal  incline  being  about  one-fourth  longer  than  the  mesial._ 
This  surface  is  generally  of  greater  longitudinal  than  transverse 
extent,  its  greatest  mesiodistal  diameter  being  from  angle  to 
angle,  or  at  a  point  immediately  above  them.  The  surface 
is  convex  in  every  direction,  and  is  marked  by  a  central  lon- 
gitudinal ridge,  usually  well  defined — the  labial  ridge.  Begin- 
ning at  the  summit  of  the 
cusp,  this  ridge  is  more  or  less 
contracted  laterally,  but  as  it 
passes  over  the  surface  in  the 
direction  of  the  rootit  becomes 
broadened  and  flattened,  and 
gradually  disappears  in  the 
cervical  portion.  Upon  either 
side  of  this  ridge  are  the 
labial  grooves,  well  defined 
at  their  beginning,  but  which 
gradually  blend  into  the  sur- 
face of  the  crown  as  they 
pass  rootward.  In  some  in- 
stances these  grooves  are  so 
strongly  defined  as  to  form  a 
decided  ridge  upon  the  mesial 

and  distal  margins  of  the  surface  ; — these  are  the  labial  marginal 
ridges. 

The  labial  ridge  and  the  two  labial  grooves  mark  the  develop- 
mental lines  of  the  crown,  the  former  resulting  from  the  middle 
lobe,  which  in  this  tooth  is  much  the  largest  of  the  three,  while 
the  latter  denotes  the  line  of  junction  between  the  middle  and 
the  lateral  lobes. 

The  Lingual  Surface  of  the  Crown  (Fig.  67). — This  sur- 
face presents  nearly  the  same  general  outlines  as  the  labial, 
with  the  exception  of  the  cervical  portion,  which  is  more  con- 
stricted, tending  to  produce  an  oblong  or  egg-shape.  It  usually 
abounds  in  well-defined  ridges  and  depressions,  giving  to   the 


Cervical   ^S 
Ridge     ^H 

■ 

Line 

Mesial   ^V 
Angle   ^m 

M 

Mesial  Cut-   ^^L 
ting-edge     ^^^^ 

i 

Distal 
Angle 
Distal  Cut- 
ting-edge 

Labial  Ridge 

Fig.  66. — Upper  Cus 

piD,  Labial 

Surface. 

156 


ANATOMY. 


tooth  a  rugged  and  strong  appearance.  There  is  but  little  gen- 
eral concavity  to  the  surface  in  passing  from  the  point  of  the 
cusp  to  the  root,  while  it  may  be  flat,  concave,  or  convex. 
As  in  the  incisors,  the  margins  of  this  surface  are  formed  by 
three  marginal  ridges  and  by  the  cutting-edge.  The  mesio- 
niarginal  ridge  is  commonly  a  well-defined  fold  of  enamel,  be- 
ginning at  the  mesial  angle  and  passing  upward  in  the  direction 
of  the  root,  where  it  unites  with  the  cervicomarginal  ridge.  It 
is  sometimes  quite  narrow  and  rather  sharply  outlined  ;  at  others, 

it  extends  well  toward  the 
center  of  the  surface  in  the 
form  of  a  well-rounded  fold. 
The  distomarginal  ridge, 
which  is  somewhat  shorter 
than  the  mesial,  begins  at  the 
distal  angle  and  passes  root- 
ward  to  meet  the  cervical 
ridge.  It  is  well  rounded  in 
every  direction,  but  seldom 
so  well  produced  as  the  me- 
sial. The  cervicomarginal 
ridge,  which  is  formed  by  a 
continuation  or  union  of  the 
two  former,  nearly  always 
partakes  of  their  nature,  ex- 
cept when  broken  by  the 
presence  of  a  cuspule,  which 
is  frequendy  found  upon  this  tooth  (Fig.  70).  This  small  cusp  of 
enamel  may  be  bounded  on  one  or  both  sides  by  a  fissure,  which 
often  extends  well  under  the  cervicomarginal  ridge,  and  some- 
times completely  separates  it  from  the  two  lateral  ridges.  Pass- 
ing through  the  center  of  the  surface  from  the  summit  of  the 
cusp  to  the  base  of  the  cervicomarginal  ridge  is  the  lingual 
ridge,  which  corresponds  to  the  labial  ridge  of  the  labial  surface. 
This  ridge  is  usually  well  produced  at  or  near  the  point  of  the 
cusp,  and  may  continue  so  throughout,  but  most  frequently 
becomes  reduced  in  size  near  the  center  of  the  surface.     Be- 


FlG.    67. 


Lingual  Ridge 
-Upper  Cuspid,  Lingual  Surface. 


UPPER   CUSPID. 


157 


tween  this  ridge  and  the  mesio-  and  disto-marginal  ridges  are 
two  longitudinal  depressions — the  lingual  grooves. 

Mesial  Surface  of  the  Crown  (Fig.  68). — In  general  out- 
line this  surface  resembles  that  of  the  central  incisor,  excepting 
that  the  wedge  shape  which  it  describes  is  more  heavily  set  and 
blunt,  with  the  surface  extending  beyond  the  base  of  the  cone, 
in  the  direction  of  the  root,  to  the  extent  of  about  one-third  of 
its  entire  length.  In  some  cases  the  base  of  the  cone  will  be 
on  a  line  with  the  cervical  margin.  The  lower  two-thirds  of 
the  surface,  or  that  nearest  the  mesial  angle,  is  convex  in  every 
direction  ;  this  convexity  grad- 
ually disappears  as  the  center 
is  approached,  beyond  which 
point  it  is  much  flattened,  usu- 
ally ending  in  a  slight  con- 
cavity at  the  cervical  margin. 
When  looking  directly  upon 
this  surface,  its  margins  will  be 
found  within  the  profile  lines, 
these  being  represented  by  the 
labial  ridge  anteriorly,  and  by 
the  lingual  and  cervical  ridges 
posteriorly.  The  margins, 
three  in  number,  are  the  labial, 
which  is  well  rounded  and 
poorly  defined ;  the  lingual, 
more  or  less  distinctly  out- 
lined and  somewhat  irregular ;  and  the  cervical,  which  is 
represented  by  the  extent  of  the  enamel  covering  of  the 
crown  ;  this  latter  margin  being  concave  in  the  direction  of  the 
root.  The  most  prominent  point  of  this  surface  serves  as  a 
point  of  contact  for  the  distal  surface  of  the  lateral  incisor,  the 
extent  of  contact  being  much  influenced  by  the  type  of  tooth, 
but  in  the  cuspid  this  is  usually  a  single  point  rather  than  an 
extent  of  surface. 

The  Distal  Surface  of  the  Crown  (Fig.  69). — This  surface 
in  many  respects  is  similar  to  the  mesial,  particularly  in  its  gen- 
eral outline.     The  extent  of  surface  is  somewhat  less  and  the 


Fig.  68.- 


-Right  Upper  Cuspid,  Mesial 
Surface. 


iS8 


ANATOMY. 


convexity  much  more  marked  than  that  of  the  mesial  surface. 
The  position  of  the  distal  angle,  which  is  the  lower  boundary 
of  the  surface,  being  much  nearer  the  cervical  line,  makes  this 
surface  about  one-third  shorter  than  the  mesial  surface.  The 
lateral  margins  of  the  surface,  which  are  also  within  the  profile 
lines,  differ  from  those  of  the  mesial  in  being  more  clearly  de- 
fined. The  cervical  margin  differs  from  that  of  the  mesial  sur- 
face by  having  a  concavity  with  much  less  depth.  As  stated 
above,  the  surface  is  decidedly  more  convex  than  the  mesial, 
the  point  of  contact  for  the  mesial  surface  of  the  first  bicus- 
pid being  almost  in  the 
center.  Near  the  cervi- 
cal margin  the  surface  is 
inclined  to  flatness,  and 
frequently  concave. 

The  Ctitting-edge,  or 
Cusp. — As  inferred  in  the 
beginning  of  this  descrip- 
tion, the  cuspid  tooth  is 
both  an  incisingand  a  pene- 
trating organ,  the  latter 
function  being  provided  for 
by  the  presence  of  the  sin- 
gle cusp,  which  divides  the 
cutting-edge  into  an  ante- 
rior or  mesial  portion  and  a 
posterior  or  distal  portion. 
The  mesial  cutting-edge 
begins  at  the  summit  of  the  cusp  and  slopes  away  to  meet 
the  mesial  angle,  which  it  assists  in  forming.  The  outline  of 
this  edge  is  usually  gracefully  curved  and  unbroken  unless 
permanently  crossed  by  the  labial  groove.  The  distal  cut- 
ting-edge is  generally  somewhat  longer  than  the  mesial. 
Immediately  after  leaving  the  summit  of  the  cusp  it  may  be 
slightly  concave  ;  but  beyond  this  point  it  is  well  rounded,  until 
it  reaches  the  distal  angle,  into  which  it  gradually  disappears. 
This  edge  is  also  frequently  broken  by  the  labial  groove.  In 
its  entirety  the  cutting-edge  is  subject  to  the  same  variations  as 


Fig.  69. — Left  Upper  Cuspid,  Distal 
Surface. 


UPPER   CUSPID. 


those  of  the  incisors — /.  e.,  it  may  be  thin  and  sharp,  or  it  may 
be  thick  and  blunt. 

The  Cusp. — The  single  cusp  from  which  this  tooth  derives  its 
name  is  formed  by  the  union  of  the  labial  ridge,  the  lingual 
ridge,  and  the  mesial  and  distal  cutting-edges.  The  summit  of 
the  cusp  is  constant  in  its  position,  always  being  in  a  direct  line 
with  the  long  axis  of  the  tooth,  whether  it  be  viewed  from  a 
mesial  or  a  lingual  direction. 

The  Cervical  Line. — To  describe  this  fully  would  be  to  repeat 
what  has  already  been  said  in  connection  with  the  incisor  teeth. 
This  enamel  margin  differs  in  one  particular  only  from  that  of 
the  incisors,  and  that  variation  is 
not  a  constant  one — the  lingual 
portion  is  frequently  extended  in 
the  direction  of  the  root,  produc- 
ing a  short,  positive  curve  at  that 
point. 

Tlie  Angles  of  the  Crow7i. — 
Owing  to  the  rounded  nature  of 
the  majority  of  cuspid  crowns,  the 
term  angle,  as  applied  to  its  free 
extremities,  is  almost  a  misnomer, 
and  can  only  be  considered  as 
assisting  in  description.  The 
mesial  angle,  which  is  formed  by 
the  union  of  the  marginal  ridges 
of  the  labial  and  lingual  surfaces 
with   the   mesial   cutting-edge,  is 

seldom  a  well-produced  angle,  usually  being  rounded  in  every 
direction.  The  distal  angle,  which  is  formed  in  a  manner  similar 
to  the  mesial,  is  somewhat  more  deserving  of  the  name,  both  the 
labio-  and  linguo-marginal  ridges  frequently  presenting  angu- 
larity. The  position  of  the  distal  angle  is  usually  well  toward  the 
center  of  the  crown,  and  occasionally  above  this  point,  and,  al- 
though it  may  descend,  it  is  seldom  found  on  a  line  with  the 
mesial  angle. 

The  Neck  of  the  Upper  Cuspid. — This  may  or  may  not 
be  a  distinctive  feature  of  the  cuspid  tooth,  although  when  viewed 


Marginal  Ridges 

Fig.  70. — Upper  Cuspid,  Lingual 
Surface,  Strongly  Developed. 


ANATOMY. 


from  a  labial  aspect,  the  lateral  flare  or  bulging  of  the  crown  gives 
the  appearance  of  a  decided  constriction  between  the  crown  and 
the  root ;  but,  when  examined  from  the  mesial  surface,  this  con- 
stricted appearance  is  absent,  the  contour  of  the  crown  passing 
into  that  of  the  root,  with  the  cervical  line  alone  marking  the  extent 
of  each.  The  tooth  at  this  point  is  well  rounded  anteriorly,  flat- 
tened laterally,  and  again  rounded  posteriorly,  the  latter  forming 
the  segment  of  a  smaller  circle  than  that  of  the  labial  surface. 

The  Root. — This  tooth  possesses  the  largest  and  longest  root 
of  any  of  the  teeth,  in  the  latter  respect  usually  exceeding  the 
central  incisor  by  about  one-third,  and  the  lateral  incisor  by  one- 
fourth  or  more.  Like  the  base  of  the  crown, 
it  is  rounded  on  the  labial  and  lingual  sur- 
faces, and  is  flattened  laterally,  this  form 
usually  being  continued  throughout  its  en- 
tire length.  It  gradually  diminishes  in  size 
from  the  neck  to  the  apex,  and  in  its 
entirety  forms  a  perfect  cone.  On  the 
mesial  and  distal  sides  it  is  not  only  much 
flattened,  but  is  frequently  provided  with  a 
longitudinal  depression,  which  is  most 
marked  near  the  center  of  its  length.  In 
some  instances  this  root  is  possessed  of  a 
slight  distal  curve,  which  may  be  gradual 
from  the  base  to  the  apex,  or  it  may  exist 
in  a  more  positive  way  by  a  sudden  distal 
curve  near  its  apical  extremity. 
Bilious  Type  (Fig.  71). — The  rounded  outlines  common 
to  the  cuspid  tooth  are  less  pronounced  in  this  type  than  in  any 
other,  and  instead  of  curves,  angles  are  present.  The  crown  is 
above  the  average  size,  length  predominating  over  breadth,  the 
cusp  well  formed,  and  the  angles  strong.  The  labial  surface 
is  often  crossed  by  a  number  of  transverse  ridges  near  the  cer- 
vical portion,  the  labial  ridge  is  bold,  as  are  also  the  labio- 
marginal  ridges.  The  mesial  and  distal  surfaces  possess  no  dis- 
tinguishing features,  but  the  lingual,  like  the  labial,  shows  the 
angular  nature  of  the  crown  in  having  its  margins  and  ridges 
squarely  set. 


Fig.  71. — Bilious  Type, 
Distal  Surface. 


UPPER   CUSPID. 


A  cuspule  is  more  frequently  found  in  this  type  than  any 
other,  and  sometimes  reaches  down  to  a  point  corresponding 
to  the  transverse  center  of  the  crown.  The  neck  is  moder- 
ately well  produced,  and  the  cervical  line  decidedly  V-shaped  on 
its  lateral  portions,  while  on  the  labial  and  lingual  it  takes  the 
form  of  a  broken  circle.  The  cutting-edges  are  rather  heavy 
and  square,  and  are  nearly  of  equal  length.  In  this  tempera- 
ment the  cuspid  tooth  often  partakes  of  the  form  described  in 
connection  with  the  lateral  incisor  of  the  same  type — /.  e.,  the 
absence  of  the  cutting-edge  and  one  or  both 
angles,  making  the  crown  a  perfect  cone. 

Nervous  Type  (Fig.  72). — The  crown 
is  of  much  greater  longitudinal  than  trans- 
verse extent,  the  outlines  oval  and  gracefully 
formed,  and  the  neck  is  much  constricted  from 
mesial  to  distal,  being  made  so  by  the  lateral 
flare  of  the  body  of  the  crown,  which  is  a 
distinctive  feature  of  this  type.  The  labial 
ridge  is  well  formed  near  the  summit  of  the 
cusp,  but  usually  disappears  near  the  center 
of  the  surface.  The  labiomarginal  ridges 
are  seldom  present,  and  the  surface  in  general 
is  convex  and  smooth.  The  mesial  and  distal 
surfaces  show  a  pronounced  convexity  near 
the  angles,  and  often  a  slight  concavity  be- 
tween this  point  and  the  cervical  line.  The  lingual  surface,  while 
generally  showing  all  the  descriptive  lines,  may  be  considered 
smooth  ;  it  is  convex  from  mesial  to  distal,  and  slightly  concave 
in  the  direction  of  the  long  axis  of  the  tooth.  The  cusp  is  long 
and  penetrating,  the  distal  cutting-edge  is  much  longer  than  the 
mesial,  and  both  are  inclined  to  sharpness.  The  cervical  line  on 
the  labial  and  lingual  surfaces  is  deeply  arched,  frequently  giving 
to  the  gingival  margin  a  receded  appearance. 

Sanguineous  Type  (Fig.  ']t^. — The  crown  in  this  type 
abounds  in  long  curves,  the  longitudinal  and  transverse  extents 
are  nearly  equal,  the  angles,  owing  to  their  circular  form,  are 
barely  deserving  the  name,  while  the  cusp  and  cutting-edges 
are  outlined  by  one  long,  oval  sweep.     The  labial  surface  is 


Fig.  72. — Nervous 
Type,  Labial-  Sur- 
face. 


prominent  and  convex,  and  the  developmental  grooves  are  fairly 
well  shown.  The  mesial  and  distal  surfaces  show  a  moderate 
general  convexity,  while  the  lingual  abounds  in  well-rounded 
ridges  and  borders.  The  constriction  forming  the  neck  of  the 
tooth  is  moderate.  The  cervical  line  is  in  the  form  of  perfectly 
arched  curves,  forming  on  the  labial  sur- 
face the  segment  of  a  circle  correspond- 
ing to  the  circumference  of  the  crown 
of  the  tooth. 

Lymphatic  Type. — In  this  type  the 
crown  is  usually  greater  in  its  transverse 
than  in  its  longitudinal  measurement;  it  is 
lacking  in  graceful  outline,  and  may  best 
be  described  as  being  short,  thick,  and 
heavy  set.  None  of  the  surfaces  abound 
in  descriptive  lines,  although  transverse 
ridges  are  sometimes  present  on  the 
labial  surface  near  the  cervix.  Both  the 
labial  and  lingual  surfaces  are  convex 
in  every  direction,  while  the  mesial  and 
distal  are  inclined  to  flatness.  The  cusp 
is  heavy  and  blunt,  and  the  cutting-edges, 
which  are  nearly  of  equal  length,  are 
thick  and  dull.  The  mesial  and  distal 
angles  are  well  produced.  The  cervical  line  is  almost  a  direct 
line  encircling  the  neck  of  the  tooth,  the  segmental  form  on  the 
labial  surface  being  that  of  a  much  larger  circle  than  any  of 
those  previously  described.  The  neck  is  thick  and  heavy,  and 
the  roots  generally  short. 


Fig.  73. — Sanguineous 
Type,  Distal  Surface. 


UPPER   FIRST   BICUSPID. 


163 


UPPER    FIRST    BICUSPID. 


Calcification  Begins,  from  Four  centers,  about  the  Fourth  Year. 
Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupted,  Tenth  to  Eleventh  Year. 
AvER.\GE  Length  of  Crown,  .32. 

Average  Length  of  Root,  .48. 

Average  Length  over  All,  .So. 


This  tooth,  although  presenting  a  crown  of  vastly  different 
contour,  is  developed  by  a  process  almost  identical  with  that  of 
the  incisors  and  cuspids.  As  the  name  implies,  it  is  made  up 
of  two  cusps,  one  forming  the  buccal  and  the  other  the  lingual 
half  of  the  crown.  Calcification  in  the  buccal  cusp  is  from  three 
centers  and  begins  about  the  fourth  year,  the  central  lobe  first 
receiving  the  lime  salts.  During  the  following  year  the  two 
lateral  lobes  begin  to  calcify,  soon  followed  by  a  union  of  the 
three,  thus  completing  the  margins  and  summit  of  the  cusp. 
Unlike  the  incisors,  but  corresponding  to  the  cuspid,  the  middle 
lobe  is  much  the  largest  of  the  three,  frequently  forcing  the 
developmental  (buccal)  grooves  well  toward  the  angles  of  the 
crown.  The  development  of  the  lingual  cusp  corresponds  to 
the  development  of  the  cervical  ridge  on  the  incisors  and  cus- 
pids, except  that  it  has  a  separate  center  of  calcification,  and 
a  cusp  almost  as  large  as  the  buccal  results.  Between  the 
fifth  and  sixth  year  union  between  the  two  cusps  takes  place, 
the  line  of  confluence  being  permanently  recorded  by  a  well- 


1 64  ANATOMY. 

defined  groove,  which  traverses  the  crown  from  mesial  to  distal. 
This  groove,  although  forced  to  occupy  a  different  position,  cor- 
responds with  the  lingual  groove  of  the  incisors  and  cuspids. 
After  the  union  of  the  cusps  the  process  of  calcification  is  con- 
tinued into  the  body  of  the  crown,  and  by  the  seventh  year  it  is 
more  than  half  completed.  The  eighth  year  usually  finds  the 
crown  fully  formed  and  the  base  of  the  root  or  roots  outlined. 
As  this  tooth  is  generally  provided  with  two  roots,  the  first  indi- 
cation of  bifurcation  will  be  observed  between  the  eighth  and 
ninth  year  by  a  filling-in  near  the  center  of  the  mesial  and  distal 
walls,  which  finally  become  united  by  a  thin  septum  of  dentine  or 
cementum.  After  this  period  the  roots  calcify  separately,  and  by 
the  middle  of  the  ninth  year  about  one-third  of  their  length  is 

established.    During  the  follow- 
Buccai  Triangular  Ridge  iug  year,  or  at  about  the  time 

of  eruption,  the  development  of 
the  roots  has  extended  to  about 
three-fourths  of  their  com- 
pleted length,  and  between  the 
eleventh  and  twelfth  year,  or 
at  a  time  corresponding  to  that 
of  the  crown  assuming  its  final 

Lingual  Cusp  ...  . 

,,,,,„„       -D  position   in  the  arch,  calcifica- 

FiG.  75. — Upper  First  Bicuspid,  r  ' 

Occlusal  Surface.  tion  is  completed  and  the  apical 

foramen  formed  (Fig.  74). 

The  Crown  of  the  Upper  First  Bicuspid  presents  for 
examination  five  surfaces — buccal,  lingual,  mesial,  distal,  and 
occlusal.  In  general,  the  contour  of  the  crown  is  irregularly 
quadrilateral,  being  about  one-third  greater  in  its  buccolingual 
measurement  than  from  mesial  to  distal.  It  is  somewhat  flat- 
tened from  mesial  to  distal,  but  rounded  on  its  buccal  and 
lingual  surfaces. 

The  Occlusal  Surface  of  the  Crown  (Fig.  75). — The 
contour  of  the  crown  is  best  observed  by  a  view  of  this  surface, 
which  may  be  described  as  trapezoidal  or  irregularly  quadri- 
lateral in  form.  The  four  margins  of  the  surface  are  those 
which  represent  the  four  lateral  surfaces, — the  buccal,  lingual, 
mesial,  and  distal, — the  latter  two  being  in  the  form  of  well- 


UPPER    FIRST   BICUSPID.  165 

defined  ridges — the  mesio-  and  disto-marginal  ridges.  The 
buccal  margin  is  formed  by  the  mesial  and  distal  inclines  of  the 
buccal  cusp ;  it  has  a  slight  buccal  convexity,  and  at  its  union 
with  the  proximate  surfaces  assists  in  forming  the  mesial  and 
distal  angles  of  the  crown.  The  distal  half  of  this  margin  is 
usually  somewhat  longer  than  the  mesial,  and  the  distal  angle 
is  less  pronounced  than  the  mesial.  The  lingual  margin  presents 
a  much  greater  convexity  than  the  buccal,  but  the  curve  formed 
is  the  segment  of  a  much  smaller  circle.  As  in  the  buccal  mar- 
gin, the  distal  half  of  this  margin  is  the  longest,  but  unlike  the 
former,  its  free  extremities  pass  into  the  mesial  and  distal  mar- 
gins without  producing  angles.  The  mesio-  and  disto-marginal 
ridges  are  strong  folds  of  enamel  which  arise  from  the  mesial 
and  distal  angles  and  converge  slightly  as  they  pass  to  the  lin- 
gual, where  they  are  gradually  lost  in  the  lingual  margin. 

The  Cusps  (Fig.  75). — These  are  two  in  number,  and  are 
named,  in  accordance  with  their  location,  buccal  and  lingual. 
The  buccal  cusp  is  the  larger  and  longer  of  the  two.  From 
the  summit  of  this  cusp  four  ridges  descend — one  in  a  mesial 
direction,  forming  the  mesial  cutting-edge  of  the  crown  ;  one  in  a 
distal  direction,  forming  the  distal  cutting-edge  ;  one  to  the  buc- 
cal surface,  the  buccal  ridge  ;  and  a  fourth,  the  btucai  triangidar 
ridge,  descends  the  central  incline.  The  mesial  and  distal  ridges 
enter  into  the  formation  of  the  mesial  and  distal  angles  at  their 
extremities  ;  the  latter  is  slightly  longer  than  the  former,  and  both 
are  frequently  broken  near  the  center  by  the  grooves  of  devel- 
opment— the  buccal  grooves.  The  buccal  ridge  may  be  well 
developed  and  extend  almost  to  the  cervical  line,  or  it  may  be 
slight  and  disappear  near  the  center  of  the  surface.  The  buccal 
triangular  ridge  usually  ends  somewhat  abruptly  in  the  central 
groove,  but  in  some  instances  it  is  continued  and  joins  a  similar 
ridge  from  the  lingual  cusp,  this  union  forming  the  transverse 
ridge.  The  triangular  ridge  often  bifurcates  near  the  center  of 
its  incline,  and  is  continued  in  two  distinct  but  smaller  ridges. 
The  lingual  cusp  is  much  less  angular  than  the  buccal  ;  the  apex 
is  usually  rounded,  while  the  descending  ridges  are  generally 
three  in  number  instead  of  four.  The  mesial  and  distal  ridges 
are  nearly  of  the  same  length,  and  pass  without  interruption  into 


1 66  ANATOMY. 

the  mesio-  and  disto-marginal  ridges.  The  triangular  ridge  is 
less  clearly  defined  than  its  fellow  of  the  buccal  cusp,  and  it  is 
not  unusual  for  it  to  be  entirely  wanting.  The  lingual  aspect  of 
the  cusp  is  smooth  and  rounded,  presenting  nothing  in  the  form 
of  a  ridge  in  correspondence  with  the  buccal  ridge  of  the  buccal 
cusp.  Like  the  incisors  and  cuspids,  the  summits  of  these  cusps 
are  usually  in  a  direct  line  with  the  long  axis  of  the  tooth. 

The  developmental  grooves,  all  of  which  are  observed  upon 
the  occlusal  surface,  are  the  central,  mesial,  distal,  two  trian- 
gular, and  two  buccal.  The  central  groove  is  the  most  marked, 
is  deeply  sulcate,  and  extends  through  the  center  of  the  sur- 
face from  mesial  to  distal,  ending  just  within  the  two  marginal 
ridges  in  two  irregularly  formed  depressions  or  pits — the  mesial 
and  distal  pits.  This  groove  marks  the  line  of  union  between 
the  buccal  and  lingual  lobes.  The  mesial  and  distal  grooves 
are  not  always  well  defined,  but  may  usually  be  observed  as 
fine  lines  passing  over  the  central  portion  of  the  mesio-  and 
disto-marginal  ridges.  The  mesio-  and  disto-triangular  grooves 
begin  in  the  mesial  and  distal  pits,  and  pass  in  the  direction  of  the 
mesial  and  distal  angles,  where  they  are  either  lost,  or  may  be 
traced  as  slight  depressions  passing  over  the  buccal  ridges 
near  the  angles,  and  they  may  further  continue  over  the  buccal 
surface  in  the  direction  of  the  root.  These  two  grooves,  together 
with  the  mesial  and  distal  above  referred  to,  form  the  outlines 
of  the  mesiobuccal  and  distobuccal  developmental  lobes.  The 
buccal  grooves  will  be  described  in  connection  with  the  buccal 
surface.  Supplemental  grooves  are  seldom  found  in  connection 
with  the  buccal  half  of  the  occlusal  surface,  but  are  occasionally 
present  on  the  central  incline  of  the  lingual  cusp. 

The  Buccal  Surface  of  the  Crown  (Fig.  76). — In  many 
respects  this  surface  resembles  the  corresponding  or  labial  sur- 
face of  the  cuspid  tooth.  It  is  bounded  by  four  margins — occlu- 
sal, mesial,  distal,  and  cervical.  The  occlusal  half  of  the  sur- 
face is  formed  of  the  buccal  cusp,  and  is  cone-shaped,  while  the 
cervical  half  is  irregularly  quadrilateral  in  form.  The  extent  of 
surface  from  the  cervical  line  to  the  point  of  the  cusp  is  usually 
about  one-third  greater  than  the  greatest  mesiodistal  diameter, 
which  is  represented  by  a  line  drawn  from  the  mesial  to  the  dis- 


UPPER    FIRST   BICUSPID. 


167 


Ridge 


tal  angle.     The  form  shown  is  that  of  a  general  convexity,  the 
summit  of  which  is  surmounted  by  a  longitudinal  ridge — the 
bticcal  ridge.     This  ridge,  which  is 
formed   from  the  central  develop- 
mental   lobe,  is  most  pronounced 
near  the  occlusal  margin,  and  grad- 
ually disappears    near   the  center 
of  the  surface.     Upon  either  side 
of  the  buccal  ridge  are  two  grooves 
— the  buccal  grooves — which  denote 
the  line  of  union  between  the  central 
and  the  two  lateral  lobes,  and  be- 
yond these  are  the  angles  of  the    ■^"s'*' 
crown.     The  buccal  ridge  springs 

from  the  buccal  cusp,  the  summit  of 

,  .    ,     .  11      •  1-  1-  Fig.  76. — Upper  First  Bicuspid, 

which  IS  generally  m  a  direct  line  Buccal  Surface. 

with    the  long  axis  of  the  tooth ; 

when    there    is    a    deviation    from   this    the   summit  is   usually 

thrown    a    little    to    the    mesial,    resulting    in    a    reduction    of 

the  length  of  the  mesial  cutting- 
edge. 

As  previously  stated,  the  great- 
est mesiodistal  diameter  of  the 
surface  is  on  a  line  with  the  angles 
of  the  crown  ;  this  measurement 
is  much  reduced  at  the  cervical 
line,  so  that  the  point  of  contact 
with  adjoining  teeth  is  thrown 
near  the  occlusal  margins  of  the 
crown.  This  variation  in  the 
transverse  measurement  also  re- 
sults in  what  is  commonly  referred 
to  as  the  "bell  shape"  of  the 
crown.  The  cervical  margin  of 
the   surface   is  fairly  well  arched, 

but  seldom  to  such  a  degree  as  the  corresponding  margin  upon 

the  incisors  and  cuspids. 

The  Lingual  Surface  of  the  Crown  (Fig.  ']•]). — This  surface 


Summit  of  Lingual  Cusp 

Fig.  77. — Upper  First  Bicuspid, 
Lingual  Surface. 


is  smooth  and  decidedly  convex,  the  absence  of  strongly  devel- 
oped grooves  and  ridges  contributing  to  the  former  fact.  Like 
the  buccal  surface,  its  greatest  transverse  measurement  is  at  the 
base  of  the  cusp  in  which  it  terminates.  The  extent  of  the  sur- 
face is  about  one-third  less  than  that  of  the  buccal,  and  its  occlud- 
ing and  cervical  margins  are  alone  well  defined,  the  mesiodistal 
convexity  passing  so  gradually  into  these  respective  surfaces 
that  a  positive  line  of  distinction  can  scarcely  be  recognized. 
In  passing  from  the  cervical  line  to  the  occlusal  margin,  the 
surface    is    rapidly    carried    toward    the    center  of  the  crown. 

The  cervical  margin  of  this 
surface  is  usually  in  the  form 
of  a  direct  line  encircling  the 
neck,  but  occasionally  pre- 
sents a  slight  concavity  in 
the  direction  of  the  root. 

The  Mesial  Surface  of 
the  Crown  (Fig.  78). — This 
surface  of  the  crown  has 
three  of  its  borders  well  de- 
fined ;  these  are  the  buccal, 
the  occlusal,  and  the  cervi- 
cal, the  remaining  or  lingual 
margin  passing  so  gradually 
into  the  lingual  surface  that 
no  positive  line  of  demarcation  can  be  given.  The  buccal  margin 
extends  from  the  mesial  angle  to  the  cervical  line,  and  invariably 
presents  a  slight  buccal  inclination,  thus  Increasing  the  width  ot 
the  surface^on  its  cervical  portion.  The  occlusal  margin  is  formed 
by  the  mesiomarginal  ridge,  and  by  a  portion  of  the  ridge 
descending  from  the  lingual  cusp.  It  is  irregularly  V-shaped, 
and  in  many  instances  is  broken  in  the  center  by  the  mesial 
groove.  The  cervical  margin  differs  from  those  of  the  incisors 
and  cuspids,  nearly  always  being  in  the  form  of  a  straight  line 
from  buccal  to  lingual.  The  surface  in  general  is  flattened,  but 
shows  a  slight  general  convexity  near  the  occlusal  margin, 
and  frequently  a  slight  concavity  immediately  below  the  cervical 
line,  this  form  placing  the  point  of  contact  with  the  distal  surface 


ginal  Ridge 


Fig.  78. — Upper  First  Bicuspid,  Mesial 
Surface. 


UPPER    FIRST    BICUSPID. 


169 


of  the  cuspid  near  the  occlusal  margin.  The  surface  is  occa- 
sionally divided  into  a  buccal  and  a  lingual  portion  by  the  mesial 
groove,  which  may  extend  to  the  cervical  line,  but  which  gener- 
ally disappears  near  the  center  of  the  surface.  The  buccal  half 
of  the  surface  which  is  formed  from  the  mesial  developmental 
lobe  is  inclined  to  angularity,  while  the  lingual  half  is  decidedly 
rounded,  particularly  in  the  direction  of  the  occlusal  margin. 

The  Distal  Surface  of  the  Crown  (Fig.  79). — In  general, 
this  surface  resembles  the  mesial,  being  flattened  and  bounded 
by  three  more  or  less  distinct 
margins.  The  slight  bucco- 
lingual  convexity  is  not  con- 
fined to  the  occlusal  portion 
of  the  surface,  but  is  inclined 
to  extend  to  the  cervical  mar- 
gin, in  this  particular  being  at 
variance  with  the  mesial  sur- 
face. This  surface  passes 
into  the  lingual  by  a  much 
longer  curve  than  that  shown 
on  the  mesial  surface. 

The  Angles  of  the 
Crown. — These  are  two  in 
number  and,  as  in  the  teeth 
previously  described,  are 
named,  according  to  their 
location,    mesial  and    distal. 


Occlusal  Surface 

79. — Upper  First  Bicuspid,  Distal 
Surface. 


The 


mesial  angle   is   formed   by 
ridore   and    mesial    cuttingr- 


the    union    of    the    mesiomarginal         _  ^ 

edge.  It  is  primarily  the  product  of  the  mesial  developmental 
lobe,  and  is  usually  well  produced.  The  distal  angle,  which  is 
formed  in  a  like  manner,  is  inclined  to  be  more  rounded. 

The  Neck  of  the  Tooth. — In  most  typal  forms  the  neck 
of  the  upper  first  bicuspid  is  well  defined,  particularly  upon 
the  mesial  and  distal  surfaces.  Viewing  the  tooth  from  a  buccal 
aspect,  the  neck  is  a  distinctive  feature,  but  when  studied  from 
the  mesial  or  distal  sides,  the  constriction  is  scarcely  observed, 
this  being  particularly  the  case  if  the  tooth  has  but  a  single  root. 
In  general,  the  neck  partakes  of  the  contour  of  the  crown,  being 


170  ANATOMY. 

convex  on  the  buccal  and  lingual,  and  flattened  and  frequently 
slightly  concave  on  the  mesial  and  distal. 

The  Roots  of  the  Upper  First  Bicuspid. — This  tooth 
is  usually  developed  with  two  roots,  sometimes  with  only  one, 
and  in  rare  instances  it  may  have  three.  When  two  roots  are 
present,  one  is  above  the  buccal  and  the  other  above  the  lingual 
half  of  the  crown,  and  are  named,  according  to  their  location, 
as  buccal  and  lingual.  In  general  form  the  two  roots  are  quite 
similar,  but  the  buccal  is  usually  a  trifle  longer  than  the  lin- 
gual. They  taper  off  to  a  slender  apex,  and  are  inclined  to 
curve  in  various  directions  near  their  extremities.     The  point  of 


Fig.  8o. — Types  of  Bicuspids. 

bifurcation  is  frequently  some  distance  above  the  neck,  so  that 
the  tooth  may  be  said  to  possess  a  single  root  with  two 
branches.  Below  the  bifurcation  the  root  assumes  the  form  of 
the  neck  or  cervical  portion  of  the  crown,  but  as  the  bifurcation 
is  approached,  the  mesial  and  distal  sides  present  a  longitudinal 
groove,  which  gradually  increases  in  depth  until  the  single  root 
becomes  separated.  The  curves  of  the  root-branches  above 
referred  to  are,  in  the  buccal  branch,  first  to  the  buccal  and 
then  to  the  lingual ;  while  the  lingual  branch  first  shows  a  slight 
lingual  inclination  immediately  above  the  point  of  separation,  fol- 


UPPER   FIRST   BICUSPID. 


lowed  by  a  gentle  buccal  curve  as  the  apical  end  is  reached.  In 
some  cases  the  bifurcation  begins  immediately  above  the  neck  of 
the  tooth ;  in  others  it  may  occur  in  the  apical  third ;  while  a  third 
class  is  represented  by  the  two  roots  being  united  throughout 
their  entire  length  by  a  thin  septum  of  dentine  and  cementum,  or, 
as  occasionally  happens,  by  a  layer  of  cementum  alone.  In  this 
latter  instance  each  root  is  provided  with  a  distinct  canal  and 
foramen.  When  the  tooth  has  but  a  single  root,  it  is  much  flat- 
tened from  mesial  to  distal,  the  flatness  being  slightly  broken 
by  an  inclination  to  convexity.  The  four  surfaces  usually  con- 
verge toward  the  apical  end,  which  is  oblong  from  buccal  to 
lingual,  and  generally  provided 
with  a  slight  distal  curve.  The 
presence  of  three  roots  is  so 
rare  that  the  condition  might  be 
classed  as  a  malformation  ;  but 
when  they  do  exist,  two  are 
usually  attached  to  the  buccal 
and  one  to  the  lingual  half  of 
the  crown,  with  the  point  of 
separation  near  the  neck  of  the 
tooth. 

Bilious  Type  (Fig.  8i).— 
The  upper  first  bicuspid  of  this 
temperamental  type  is  marked 
by  a  crown  of  moderate  length, 

the  neck  well  pronounced,  and  the  cusps  and  angles  marked 
by  angular  outlines.  The  buccal  ridge  is  strongly  defined,  and 
the  buccal  grooves,  which  extend  well  up  on  the  buccal  surface, 
cross  the  mesial  and  distal  cutting-edges,  separating  them  into 
two  distinct  parts.  The  cusps  are  long  and  penetrating,  and 
are  nearly  of  equal  length,  assisting  to  form  the  firm  and  well- 
locked  occlusion  common  to  this  type.  The  mesial  and  distal 
surfaces  are  nearly  parallel  with  each  other ;  they  are  seldom 
convex,  so  that  the  approximating  teeth  are  in  contact  over  an 
extent  of  surface  rather  than  a  single  point.  The  cervical  line 
is  but  little  curved. 

Nervous    Type   (Fig.  82). — In  this  temperament  the  bell- 


Si. — Bilious  Type,  Distal 
Surface. 


shaped  crown  Is  strongly  observed,  die  crown  being  long  and  the 
tooth  much  constricted  at  its  neck.  The  extreme  length  of  the 
buccal  cusp,  and  the  marked  cervical  constricdon,  produce  an 
appearance  in  the  buccal  surface  resembling  the  labial  surface  of 
the  cuspid  tooth.  The  developmental  grooves  are  finely  outlined, 
and  the  cusps  long  and  penetrating,  usually  being  more  pro- 
nounced than  in  any  other  class.  The  cutdng-edges  are  sharp 
and  inclined  to  angularity ;  the  mesial  and  distal  surfaces  are 
convex  near  their  occlusal  margins,  but  near  the  cervical 
line  a  pronounced  concavity  is  observed,  which  is  continued 
upon  the  corresponding  root-surfaces.  This  formation  forces 
the  point  of  contact  with  adjoining  teeth  well 
toward  the  occlusal  surface,  and  results  in  an 
extensive  V-shaped  interproximate  space.  The 
cervical  line  is  sharply  and  gracefully  formed, 
the  curvature  being  well  arched. 

Sanguineous  Type. — The  typical  upper 
first  bicuspid  of  this  class  is  provided  with 
a  crown  well  proportioned,  its  length  being 
somewhat  greater  than  its  breadth,  but  about 
equal  to  its  buccolingual  measurement.  The 
buccal  surface  is  seldom  broken  by  the 
buccal  grooves,  and  is  strongly  convex  in 
every  direction.  The  lingual  surface  is  much 
more  rounded  than  the  same  surface  of  other 
types.  The  mesial  and  distal  surfaces  are 
usually  smoothly  convex,  with  an  occasional  slight  concavity 
immediately  below  the  cervical  line.  Upon  the  occlusal  surface 
the  grooves  are  rounded  and  obscure,  rather  than  sharp  and  well 
defined,  and  the  cusps,  much  less  pronounced  than  in  either  of  the 
types  previously  described,  are  rounded  and  smooth  ;  this  latter 
fact  is  particularly  true  of  the  lingual  cusp,  which  is  usually  much 
smaller  than  the  buccal.  The  cutting-edges  of  the  buccal  cusp 
are  scarcely  deserving  of  the  name,  being  broad  and  rounded 
throughout.  The  form  of  the  mesial  and  distal  surfaces  above 
described  provides  for  a  point  of  contact  near  the  center  of 
each  surface,  leaving  a  slight  interproximate  space  both  above 
and  below  this  point. 


Fig.  82. — Nerv- 
ous Type,  Buccal 
Surface. 


UPPER   FIRST   BICUSPID.  173 

Lymphatic  Type. — An  examination  of  the  upper  first  bi- 
cuspid of  the  lymphatic  type  results  in  finding  a  tooth  vastly 
different  from  any  of  those  previously  described.  The  length 
of  the  crown  from  the  cervical  line  to  the  point  of  the  cusp  is 
less  than  either  the  mesiodistal  or  buccolingual  measurements. 
In  general  appearance  it  is  lacking  in  symmetry,  or  poorly  pro- 
portioned. The  buccal  surface  presents  a  gradual  convexity 
from  mesial  to  distal,  and  seldom  has  the  buccal  ridge  well  de- 
veloped. The  lingual  surface  is  smoothly  convex  and  passes 
off  into  the  lingual  root  without  the  interposition  of  a  decided 
neck.  The  mesial  and  distal  surfaces  are  flattened  and  spar- 
ingly conve.x,  and  are  nearly  parallel  with  each  other,  so  that 
the  contact  with  adjoining  teeth  is  inclined  to  be  distributed 
over  the  entire  surface,  leaving  little  or  no  interproximate  space. 
The  cusps  are  short,  flat,  and  rounded,  and  the  occlusal  sur- 
face much  flattened  in  general,  corresponding  with  the 
nature  of  the  occlusion,  which  is  loose  and  wandering.  The 
developmental  grooves  and  ridges  are  fairly  well  shown,  while 
the  cutting-edges  and  angles  of  the  crown  are  smooth  and 
rounded.  The  neck  is  less  pronounced  in  this  type  than  in  any 
other,  the  curvature  of  the  cervical  line  is  very  slight,  the  root 
is  short  and  heavy  set,  frequently  passing  well  up  toward  the 
apex  before  bifurcating. 


UPPER    SECOND    BICUSPID. 


9  §  a 


Calcification  Begins,  from  four  Centers,  about  the  Fifth  Year. 
Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupted,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .29. 

Average  Length  of  Root,  .55. 

Average  Length  Over  All,  .84. 

The  process  of  development  in  this  tooth  is  identical  with 
that  of  the  first  bicuspid,  calcification  in  the  buccal  half  of  the 
crown  taking  place  in  one  central  and  two  lateral  lobes,  while 
the  lingual  half  is  developed  from  a  single  center.  The  calcify- 
ing process  is  about  one  year  later  than  that  in  the  first  bicuspid, 
the  summit  of  the  buccal  cusp  receiving  its  lime  salts  about 
the  beginning  of  the  fifth  year.  During  the  following  six 
months  calcification  begins  in  the  lateral  lobes,  and  also  in  the 
lingual  lobe.  By  the  sixth  year  the  occlusal  surface  and  a  por- 
tion of  the  crown  are  completed  by  a  union  of  the  various  lobes, 
and  at  seven  years  the  crown  is  calcified  for  more  than  two- 
thirds  of  its  completed  length.  Between  the  eighth  and  ninth 
year  the  contour  of  the  crown  is  established,  and  the  neck  of 
the  tooth  and  outline  of  the  root-base  formed.  At  the  tenth 
year  about  one-third  of  the  root-length  is  formed,  and  during 
the  following  year  about  ^  of  an  inch  is  added  to  it.  By 
the  eleventh  or  twelfth  year  calcification  is  completed  and  the 
apical  foramen  established  (Fig.  83). 


UPPER   SECOND   BICUSPID. 


I7S 


fI3 
1^ 


Fig.  84 


— Second   Upper   Bicuspid, 
Occlusal  Surface. 


This  tooth  so  closely  resembles  the  first  bicuspid  that  a  de- 
scription in  detail  will  be  unnecessary  ;  there  are,  however,  a  few 
minor  points  which  are  at  variance  and  must  be  described  in 
order  to  distinguish  one  from  the  other.  In  general,  the  tooth 
is  a  trifle  smaller  than  the  first  bicuspid,  the  cusps  are  somewhat 
shorter,  and  the  various  ridges  less 
distinct.     A   distinguishing   feature  ce.urai 

of  the  occlusal  surface  is  found  in 
the  diminished  length  of  the  central 
groove  (Fig.  84).  This  groove,  as 
observed  in  the  first  bicuspid,  ex- 
tends from  mesial  to  distal  for  fully 
three-fourths  of  the  entire  width  of 
the  surface  ;  but  in  the  second  bicus- 
pid it  is  diminished  by  one-third,  be- 
ing thus  reduced  by  the  broadened 

marginal  ridges,  which  force  the  mesial  and  distal  pits  well 
toward  the  center.  It  is  not  uncommon  to  find  the  triangular 
grooves  joining  the  central  groove  direcdy  in  the  center  of  the 

surface,  forming  a  central  pit 
from  which  may  radiate  numer- 
oussmall  supplemental  grooves 
and  ridges.  The  summits  of 
both  the  buccal  and  lingual 
cusps  are  nearer  to  the  mesial 
than  to  the  distal  surface,  thus 
increasing  the  length  of  the 
ridges  which  descend  from  them 
in  a  distal  direction,  and  de- 
creasing those  which  pass  to  the 
mesial.  The  buccal  surface 
presents  a  greater  convexity 
than  that  of  the  first  bicuspid  ; 
the  buccal  grooves  are  usually 
shallow  depressions  and  are  fre- 
quently entirely  wanting,  thus  giving  the  buccal  ridge  the  appear- 
ance of  extending  its  margins  to  the  angles  of  the  crown.  Unlike 
the  first  bicuspid,  the  mesiodistal  diameter  of  the  mesial  surface 


Fig.  85. — Upper  Second  Bicuspid,  Me- 
sial Surface.    Most  common  form. 


176 


is  but  little  more  than  the  same  measurements  on  the  lingual 
surface.  The  neck  of  the  tooth  is  not  quite  so  pronounced  as 
that  of  the  first  bicuspid,  thus  giving  less  of  the  bell  shape  to 
the  crown.  One  very  important  difference  between  this  tooth 
and  the  first  bicuspid  is  in  the  root-formation.  We  have  seen 
that  the  first  bicuspid  is  generally  provided  with  two  roots,  while 
in  the  second  bicuspid  a  single  root  is  usually  present.  Like  the 
first  bicuspid,  there  are  exceptions  to  this,  the  tooth  sometimes 
being  provided  with  two,  and  in  rare  in- 
stances with  three,  roots.  When  the  single 
root  is  present,  it  partakes  of  the  form  of 
the  crown  at  its  base,  being  well  rounded 
on  the  buccal  and  lingual  portions  and  much 
flattened  on  the  mesial  and  distal.  The 
mesiodistal  diameter  of  the  root  at  its  base 
is  only  about  one-third  that  of  the  bucco- 
lingual  measurement,  this  proportionate 
size  continuing  throughout  its  entire  length. 
In  passing  from  the  base  to  the  apex  the 
root  is  gradually  diminished  in  size,  finally 
ending  somewhat  abruptly  in  an  oblong 
extremity.  In  some  instances  the  apical 
end  is  round  and  pointed,  resembling  the 
apex  of  the  incisors  and  cuspids,  but 
when  thus  formed  the  root  is  usually 
curved  near  its  apical  third  and  somewhat  extended  in  length. 
The  mesial  and  distal  surfaces  are  provided  with  a  well-defined 
longitudinal  concavity,  extending  from  the  cervical  margin  to  the 
apex  and  dividing  the  root  into  a  buccal  and  a  lingual  portion. 
This  depression  is  often  so  decided  that  the  contour  of  the  single 
root  is  almost  lost,  and  in  its  place  the  appearance  is  that  of  two 
roots  similar  to  those  described  in  the  first  bicuspid.  The  length 
of  the  root  is  usually  a  little  greater  than  that  of  the  first  bicus- 
pid, but  the  crown  being  a  trifle  shorter,  results  in  producing  a 
tooth  the  entire  length  of  which  is  about  equal  to  that  of  the  first 
bicuspid. 


Fig.  86.— Young  Up- 
per Second  Bicuspid, 
Lingual  Surface. 


UPPER  FIRST  MOLAR. 


UPPER    FIRST    MOLAR. 


€^  m  i9  II  K 


2d  year  3d  year 


5tli  year  6th  year  ylh  ye 

Fig.  87. 


Calcification  Begins,  from  Four  Centers,  about  One  Month  Before  Birth. 
Calcification  Completed,  Ninth  to  Tenth  Year. 
Erupted,  Sixth  to  Seventh  Year. 
Average  LengtIi  of  Crown,  .30. 

Average  Length  of  Root,  .51. 

Average  Length  over  All,  .81 

This  tooth  being  the  first  of  the  permanent  organs  to  erupt, 
it  precedes  all  others  in  the  process  of  calcification,  beginning 
to  receive  its  lime  salts  as  early  as  the  eighth  fetal  month. 
The  form  of  the  crown  being  so  entirely  different  from  those 
previously  described,  embodies  a  developmental  process  which 
is  also  different,  four  distinct  lobes  being  present,  one  for,  each 
cusp,  these  making  their  appearance  during  the  first  year  after 
birth,  closely  followed  by  a  completion  of  the  occlusal  surface 
by  the  union  of  the  free  calcifying  margins,  these  lines  of 
union  being  finally  represented  by  the  developmental  grooves 
of  the  occlusal  surface.  After  the  completion  of  this  surface, 
calcification  proceeds  in  the  direction  of  the  base  of  the  crown, 
and  at  the  beginning  of  the  third  year  about  two-thirds  of  the 
crown  is  formed.  During  the  fifth  year  the  contour  of  the 
crown  is  completed,  and  at  the  beginning  of  the  eruptive 
period,  or  about  the  si.\th  year,  the  developmental  process  has 
extended  to  the  base  of  the  roots,  and  an  effort  at  trifurcation 
begun.  At  seven  years  the  three  roots  with  which  the  tooth  is 
provided  are  branching  out,  each  into  its  own  socket,  subsequent 


178  ANATOMY. 

development  in  each  being  continued  as  a  separate  and  distinct 
process.  The  tenth  year  more  than  half  completes  the  calcifying 
process  in  the  roots,  and  at  the  beginning  of  the  eleventh  year 
the  apical  foramina  are  established  (Fig.  87).  Like  the  first  bi- 
cuspid, the  crown  of  this  tooth  presents  for  examination  five  sur- 
faces— occlusal,  buccal,  lingual,  mesial,  and  distal.  In  general 
contour  it  is  irregularly  quadrilateral,  with  the  angles  of  the 
crown  more  or  less  rounded,  two  of  its  sides  convex  and 
two  flattened  or  slightly  concave.  The  length  of  the  crown 
from  the  cervical  line  to  the  summits  of  the  cusps  is  about  equal 
to,  or  slightly  less  than,  its  mesiodistal  diameter,  while  the  bucco- 
lingual  measurement  is  usually  a  trifle  greater  than  the  mesio- 
distal. 


Central  Fossa    ^S     't^S^  |fc"^M    Distobuccal 

Oblique  Ridge    ■      ^-        ^Jf    H   DistaTFossa 

MesioIi.,gual  Cusp    M^        ^^         ■   gi'.^a?  R?dge 


The  Fifth  Cusp 


Fig.  88. — Upper  First  Molar,  Occlusal  Surface. 

The  Occlusal  Surface  of  the  Crown  (Fig.  88).— The 
coronal  outline  of  this  tooth  is  best  studied  when  looking 
directly  upon  this  surface,  which  shows  the  two  convex  sides 
above  referred  to,  represented  by  the  buccal  and  lingual  mar- 
gins, with  the  mesial  and  distal  margins  more  or  less  flat- 
tened. The  surface  is  bounded  by  these  four  margins,  which 
are  nearly  of  equal  length,  the  angles  formed  by  their  union 
being  more  or  less  rounded,  two  of  which,  the  mesiobuccal 
and  the  distolingual,  are  acute  angles,  while  the  mesiolingual 
and  distobuccal  are  obtuse  angles.  The  surface  is  divided 
into  four  developmental  portions — the  mesiobuccal,  disto- 
buccal, mesiolingual,  and  distolingual.  Each  one  of  these 
parts  is  surmounted  by  a  well-defined  point  or  cusp,  which 
likewise    is    named    in    accordance    with    its   location.       These 


UPPER    FIRST   MOLAR.  179 

various  parts  are  separated  from  one  another  by  four  develop- 
mental grooves — the  mesial,  the  buccal,  the  distal,  and  the  disto- 
lingual.  In  the  center  of  the  triangle  formed  by  the  central 
incline  of  the  mesiobuccal,  distobuccal,  and  mesiolingual  cusps 
is  a  deep  depression, — the  central  fossa, — while  near  the  distal 
margin  is  a  somewhat  similar  depression — the  distal  fossa. 
Traversing  the  surface  in  various  directions  are  a  number  of 
ridges  and  supplemental  grooves,  each  of  which  will  be  de- 
scribed in  turn. 

The  Marginal  Ridges  of  the  Occlusal  Surface. — These  are 
four  in  number — the  mesial,  distal,  buccal,  and  lingual.  The 
mesiomarginal  ridge  is  a  well-pronounced  elevation  of  enamel 
which  passes  from  the  mesiobuccal  angle  to  the  mesiolingual 
angle.  It  is  slightly  concave  in  the  direction  of  the  root,  and  is 
broken  near  the  center  of  its  concavity  by  the  mesial  groove, 
upon  either  side  of  which  are  frequently  found  one  or  two  small 
points  or  tubercles,  which  are  formed  either  by  a  division  of  the 
mesial  developmental  groove,  or  by  one  or  more  supplemental 
grooves.  These  grooves  pass  over  the  ridge  and  are  continued 
for  a  short  distance  on  the  mesial  surface.  Descending  from 
the  mesiobuccal  cusp,  the  ridge  passes  in  a  lingual  direction  to 
meet  the  mesiolingual  cusp,  and  in  so  doing  has  a  slight  distal 
inclination  until  the  mesial  groove  is  reached,  after  passing 
which  it  makes  a  sweeping  distal  curve  and  is  lost  in  the  lingual 
margin.  This  ridge  marks  the  line  of  junction  between  the 
occlusal  surface  and  the  mesial  surface.  The  distomarginal 
ridge  in  some  respects  resembles  the  mesial  just  described, 
being  concave  and  ascending  in  a  buccal  and  lingual  direction, 
with  a  somewhat  rounded  outline,  to  the  summits  of  the  disto- 
buccal and  distolingual  cusps.  The  depth  of  the  concavity  is 
usually  greater  than  that  of  the  mesial  margin,  and  is  fre- 
quently crossed  near  the  center  by  the  distolingual  groove, 
frequently  so  marked  as  to  produce  a  V-shape  to  the  center  of 
the  margin.  There  are  occasionally  found  upon  either  side  of 
this  central  groove  one  or  more  small  tubercles,  corresponding 
to  those  of  the  mesial  ridge,  but  they  are  less  frequent  and  less 
pronounced.  This  ridge  forms  the  line  of  demarcation  between 
the  occlusal  surface  and  the  distal  surface.    The  bnccomar spinal 


ridge  begins  at  the  mesiobuccal  angle,  and  gradually  ascends  to 
the  summit  of  the  mesiobuccal  cusp,  from  which  it  afterward 
descends  in  a  distal  direction  to  the  buccal  groove  ;  continuing, 
it  again  ascends  the  distobuccal  cusp,  after  descending  from 
which  it  ends  in  the  distobuccal  angle.  The  nature  of  this  ridge 
is  a  series  of  cutting-edges,  giving  to  the  cusps  their  angular 
nature.  Besides  the  buccal  groove,  which  makes  a  decided 
break  in  the  center  of  its  course,  the  ridge  is  frequently  crossed 
by  numerous  small  supplemental  grooves  occurring  in  various 
locations  and  forming  a  series  of  minute  tubercles  ;  this  latter 
condition  is  most  frequently  present  in  young  teeth,  and  is  soon 
obliterated  by  wear.  The  course  of  this  ridge  is  not  that  of  a 
direct  line  from  mesial  to  distal,  but  in  its  ascent  of  the  mesio- 
buccal cusp  it  is  inclined  to  the  buccal ;  in  its  descent  it  pre- 
sents a  corresponding  return  to  the  lingual,  and  the  same 
variations  are  observed  in  passing  over  the  distal  cusp. 
The  linguomarginal  ridge  begins  at  the  mesiohngual  angle 
of  the  crown  and  passes  distally  to  the  distolingual  angle, 
differing  from  the  three  previously  described  by  being  heavy 
and  rounded  in  its  nature,  more  irregular  in  outline,  and 
divided  nearest  to  its  distal  extremity  instead  of  in  the  center 
of  its  length.  From  the  point  of  beginning  it  makes  a  curved 
ascent  to  the  summit  of  the  mesiolingual  cusp ;  descending 
from  this  in  a  distobuccal  direction,  it  divides,  one  portion 
passing  to  join  the  triangular  ridge  of  the  distobuccal  cusp, 
the  two  uniting  to  form  the  oblique  ridge,  the  other  portion 
continuing  in  the  direction  of  the  distolingual  cusp,  before 
reaching  the  base  of  which  it  is  broken  by  the  distolingual 
o-roove.  From  this  groove  the  ridge  makes  a  sudden  and  direct 
ascent  to  the  summit  of  the  distolingual  cusp,  after  passing 
which  it  gradually  descends  in  a  long  curve  to  join  the  disto- 
marginal  ridge.  Like  the  buccal  ridge,  it  is  frequently  crossed 
by  numerous  supplemental  grooves.  The  ridge  forms  the  lin- 
gual margin  of  the  occlusal  surface,  and  gives  to  the  cusps  their 
angularity. 

The  Cusps  (Fig.  88). — These  are  four  in  number  (see 
also  Fig.  89) — the  mesiobuccal,  distobuccal,  mesiolingual,  and 
distolingual. 


UPPER   FIRST   MOLAR.  l8l 

The  Mesiobuccal  Cusp  (Fig.  88). — In  extent  of  surface  this 
is  usually  the  largest  cusp,  although  it  is  sometimes  exceeded 
by  the  mesiolingual.  From  the  summit  of  the  cusp  three  ridges 
descend — the  biucal  ridge  to  the  buccal  surface,  the  buccomar- 
ginal  ridge  making  a  double  descent,  and  the  mesiobuccal  tri- 
angular ridge,  the  latter  descending  the  central  incline  and 
ending  in  the  central  fossa.  The  mesial  base  of  the  cusp  is  fre- 
quently crossed  by  one  or  more  supplemental  grooves,  which 
begin  at  the  mesial  margin  and  pass  in  the  direction  of  the 
central  fossa.  The  central  slope  of  the  cusp  contributes  to  the 
formation  of  the  central  fossa,  its  extent  in  this  direction  being 
controlled  by  the  mesial  and  buccal  grooves,  which  together 
form  the  mesiobuccal  triangular  groove. 

The  DistobiLccal  Cusp  (Fig.  88). — This  cusp  is  frequently  the 
smallest  in  extent  of  surface,  but  is  usually  longer  and  more 
pointed  than  the  others.  Like  the  mesiobuccal  cusp,  three 
ridges  descend  from  it — the  buccal  ridge  to  the  buccal  surface 
two  which  spring  from  the  buccomarginal  ridge,  and  the  disto- 
buccal  triangular  ridge,  which  descends  obliquely  toward  the 
distal  center  of  the  surface  and  joins  a  similar  ridge  (previously 
described)  from  the  mesiolingual  cusp,  the  two  forming  the 
oblique  ridge.  The  mesial  portion  of  the  base  of  this  cusp 
assists  in  forming  the  central  fossa,  while  a  portion  of  the  distal 
contributes  to  the  formation  of  the  distal  fossa.  The  inner 
boundary  of  the  cusp  is  formed  by  the  buccal  groove,  the  distal 
groove,  and  by  a  portion  of  the  distolingual  groove. 

The  Mesiolingual  Cusp  (Fig.  88). — As  above  stated,  this 
cusp  is  frequently  the  largest  in  extent  of  surface,  and  is  some- 
what rounded,  with  its  summit  poorly  defined.  The  ridges 
which  descend  from  it  correspond  in  name  and  number  to  those 
of  the  buccal  cusps,  the  linguomarginal  ridge  making  a  double 
descent,  the  mesiolingual  ridge  descending  to  the  lingual  sur- 
face, while  the  central  incline  is  marked  by  the  mesiolingual 
triangular  ridge,  which  ends  in  the  central  fossa.  Toward  the 
mesial  portion  of  the  cusp  one  or  more  small  ridges  are  fre- 
quendy  present,  extending  from  the  marginal  ridge  to  the 
mesial  groove.  The  distal  descent  of  the  marginal  ridge  is 
bifurcated,  one  portion   making  a  sweeping  curve  and  joining 


the  transverse  ridge  from  the  buccal  cusp,  forming  the  obhque 
ridge  previously  referred  to,  the  other  portion  passing  in  a  distal 
direction  and  ending  at  the  distolingual  groove.  The  central 
incline  of  this  cusp  forms  the  lingual  side  of  the  central  fossa, 
and  its  boundaries  are  outlined  by  the  mesial,  distal,  and  disto- 
lingual grooves. 

The  Distolingual  Cusp  (Fig.  88). — This  cusp  is  usually  the 
smallest  of  the  four;  it  is  triangular  in  outline,  with  the  summit 
nearest  the  mesiolingual  portion.  The  ridges  which  descend 
from  this  cusp  are  only  two  in  number,  one  passing  in  a  mesial 
direction  and  forming  a  portion  of  the  linguomarginal  ridge, 
the  other  passing  to  the  distal,  with  a  gradual  buccal  curve,  to 
join  the  distomarginal  ridge.  Of  the  two  remaining  inclines, 
one  looks  in  a  distolingual  direction,  presenting  a  surface  which 
is  smooth  and  rounded  ;  the  other,  sloping  by  a  broad  expanse 
in  a  mesiobuccal  direction,  ending  in  the  distolingual  groove, 
and  also  assisting  to  form  the  distal  fossa.  This  latter  incline 
is  often  crossed  by  small  supplemental  grooves,  which  take  a 
winding  course  from  the  base  to  the  summit  of  the  incline.  The 
inner  margin  or  outline  of  the  cusp  is  formed  by  the  disto- 
lingual groove. 

The  Fifth  Cusp  (Fig.  89). — Although  usually  referred  to  as 
possessing  but  four  cusps,  this  tooth  is  frequently  developed 
with  five,  the  additional  lobe  being  situated  on  the  lingual  side 
of  the  mesiolingual  cusp,  about  midway  between  its  summit  and 
the  neck  of  the  tooth.  When  present,  it  is  distinctly  separate 
from  the  main  cusp  by  a  well-developed  groove — the  mesio- 
ling2ial  groove.  Both  the  cusp  and  the  groove  may  be  more  or 
less  developed,  the  former  in  some  instances  assuming  dimen- 
sions corresponding  to  that  of  the  distolingual  cusp,  and  the 
latter  sometimes  being  as  well  marked  as  the  distolingual 
groove.  When  thus  pronounced,  the  groove  begins  near  the 
center  of  the  mesial  surface,  and  passes  obliquely  toward  the 
summit  of  the  mesiolingual  cusp,  before  reaching  which  it  makes 
an  abrupt  turn  rootward,  and  joins  the  lingual  terminal  of  the 
distolingual  groove,  this  union  frequently  resulting  in  a  well- 
defined  pit — the  lingual  pit.  This  cusp,  as  usually  found,  is  small 
and  apparently  without  function.      When  occurring  on  the  tooth 


UPPER   FIRST   MOLAR.  183 

of  one  side,  it  is  usually  present  on  the  corresponding  tooth  of 
the  opposite  side.  It  is  seldom  present  on  any  but  the  upper 
first  molar. 

The  Fossae  and  Grooves  of  the  Occlusal  Surface  (Figs. 
88  and  89). — The  fosste  are  two  in  number — central  and  distal. 
The  central  fossa  occupies  a  position  near  the  center  of  the  sur- 
face, and  is  formed  by  the  central  incline  of  the  mesiobuccal, 
distobuccal,  and  mesiolingual  cusp,  which  usually  give  it  a 
three-sided  form.  Connecting  the  three  sides  of  the  fossa,  and 
in  a  measure  assisting  in  its  construction,  is  the  mesiomarginal 
ridge  and  the  oblique  ridge.  The  depth  of  this  fossa,  as  well  as 
that  of  the  distal,   is  of  course  regulated  by  the  length  of  the 


Buccal  Groove 

Mesiobuccal 

Cusp 
Central  Fossa 
Mesiomar- 
ginal Ridge 

The  Fifth 
Cusp 

E 

1 

Distobucc, 
Cusp 


Distolingt 
Cusp 


Mesiolingual  Oblique 
Cusp  Ridge 

Fig.  89.— Upper   First   Molar,  Occlusai,  Surface,    Strongly   Developed,  showing 
Presence  of  Fifth  Cusp. 

cusps,  which  in  turn  is  much  influenced  by  the  temperamental 
type  of  the  tooth.  The  bottom  of  the  fossa  is  deeply  marked 
by  two  of  the  grooves  of  development — the  mesial  groove  and 
the  buccal  groove.  The  former  begins  on  the  mesial  surface 
passes  over  the  mesiomarginal  ridge,  and  continues  in  an  irreg- 
ular line  to  the  bottom  of  the  fossa  ;  the  latter,  beginning  near 
the  center  of  the  buccal  surface,  enters  the  fossa  by  crossing  the 
buccomarginal  ridge  near  the  center  of  its  length,  and  also 
ends  in  the  central  pit  of  the  central  fossa.  i\s  previously 
referred  to,  the  union  of  these  two  grooves  forms  the  mesio- 
bnccal  triangular  groove.  From  the  central  pit  of  this  fossa 
another  groove  is  given  off — the  distal  groove.     It  is  usually  well 


i84  ANATOMY. 

defined  at  its  beginning,  but  as  it  passes  over  the  oblique  ridge 
it  is  generally  partly  obliterated,  although  occasionally  being  so 
marked  as  to  divide  this  ridge.  The  distal  fossa  is  much 
smaller  than  the  central,  and  is  of  an  entirely  different  form. 
Its  walls  are  principally  formed  by  the  distolingual  incline  of  the 
oblique  ridge,  and  the  mesiobuccal  incline  of  the  distolingual  cusp  ; 
a  portion  of  the  distomarginal  ridge  and  the  distal  incline  of  the 
distobuccal  cusp  also  assist  in  its  formation.  Like  the  central 
fossa,  its  sides  are  more  or  less  irregular,  from  the  presence  of 
various  grooves  and  ridges  in  its  vicinity.     The  greatest  length 

of  the  fossa  is  in  a  disto- 
lingual direction,  and  it  is 
traversed  by  a  deep  de- 
velopmental groove — the 
1  Root  distolingual  groove.  When 
the  distal  groove  crosses 
the  oblique  ridge,  it  usu- 
ally extends  to  the  bottom 
of  this  fossa. 

The  Buccal  Surface 
of  the  Crown  (Fig.  90). 
— This  surface,  which  is 
the  result  of  a  union  be- 
tween the  mesial  and  dis- 
Buccai  Groove  tal  developmental   lobes. 

Fig.  go.— Upper  First  Molar,  Buccal  may    be     divided    into     a 

Surface.  .    ,  ,  ,.        ,        ,^ 

mesial  and  a  distal  halt. 

These  two  portions  are 
quite  similar  in  outline,  and  are  separated  from  each  other 
by  the  buccal  groove,  which  usually  ends  near  the  center  or 
about  half-way  to  the  cervical  line  in  a  decided  pit — the 
buccal  pit.  In  some  instances  this  groove  is  continued  to 
the  cervical  line,  or  even  beyond  this  to  the  bifurcation  of 
the  roots.  Both  the  mesial  and  distal  half  are  provided  with 
a  longitudinal  ridge  (the  buccal  ridges) — one  the  mesiobuccal 
ridge  and  the  other  the  distobuccal  ridge.  These  are  similarly 
formed  and  descend  from  the  summits  of  the  respective  cusps,  at 
which  point  they  are  usually  well  defined,  but  gradually  disappear 


UPPER    FIRST   MOLAR. 


185 


as  they  pass  toward  the  cervical  hne.  The  location  of  the  buccal 
groove  being  a  little  to  the  distal  of  the  center  of  thesurface,  gives 
to  the  mesial  portion  a  somewhat  greater  extent  than  the  distal. 
The  margins  of  the  surface,  which  form  an  irregular  quadri- 
lateral, are  the  mesial,  distal,  occlusal,  and  cervical.  The  mesial 
and  distal  margins  are  rounded,  and  gradually  converge  as  they 
pass  rootward,  making  the  average  diameter  of  the  surface 
about  one-fourth  less  at  the  cervical  line  than  at  the  base  of  the 
cusps.  In  some  instances  these  margins  are  slightly  concave 
over  their  cervical  portion,  and  convex  on  approaching  the  occlu- 
sal margins  ;  or  the  mesial  may  be  concave  and  the  distal  con- 
vex throughout  their  en- 
tire length  ;  in  some  types 
they  appear  as  straight 
lines  and  are  parallel  with 
each  other.  The  occlu- 
sal margin  is  formed  by 
the  marginal  ridges  as 
they  pass  over  the  two 
buccal  cusps,  being  in  the 
form  of  the  letter  W.  The 
cervical  margin  is  usually 
a  direct  line  drawn  around 
the  circumference  of  the 
tooth,  but  in  some  in- 
stances deviating  slightly 
from    this.      Immediately 

below  the  cervical  line,  and  conforming  to  its  general  direction, 
is  a  rounded  fold  of  enamel — the  cervical  ridge. 

Lingual  Surface  of  the  Crown''=  (Fig.  91). — In  correspond- 
ence with  the  buccal  surface,  this  surface  is  developed  from  two 
lobes, — the  mesio-  and  disto-lingual  lobes, — the  line  of  union  be- 
tween the  two  being  recorded  by  a  well-defined  groove — the  lin- 
gjial groove.  This  groove,  which  is  a  continuation  of  the  distolin- 
gual  groove  of  the  occlusal  surface,  usually  ends  near  the  center 
of  the  lingual  surface  in  a  well-defined  pit — the  lingual  pit — or  it 

*When  the  fifth  cusp  is  present,  the  anatomy  of  the  mesial  half  of  this  surface  is  somewhat 
more  complex. 


Lingual     Lingual  Ridge 
Groove     Mesiolingual  Cusp 

Fig.  91. — Upper  First  Molar,  Lingual 
Surface. 


may  continue  rootward  and  gradually  disappear.  It  Is  located 
a  little  to  the  distal  of  the  center  of  the  surface,  thus  making  the 
mesial  a  trifle  larger  than  the  distal  portion.  The  mesial  half 
of  the  surface  is  smooth  and  conve.x  ;  the  lingual  incline  of  the 
mesiolingual  cusp  is  seldom  provided  with  a  well-defined  ridge, 
although  usually  referred  to  as  the  mesiolingiMl  ridge.  The 
distal  half  of  the  surface  is  also  smooth  and  rounded,  with  the 
mesiodistal  convexity  much  more  marked  than  that  of  the  mesial 
lobe.  The  cervical  ridge  is  seldom  so  pronounced  as  that  of  the 
buccal  surface,  but  the  enamel  frequently  makes  a  sudden  dip  at 
this  point  to  meet  the  cementum  of  the  root.  That  portion  of 
the  surface  immediately  below  the  cervical  ridge  is  smooth  and 

unbroken,  slightly  convex 
in  the  direction  of  the  long 
axis  of  the  tooth,  and 
flattened  or  slightly  con- 
vex from  mesial  to  distal. 
The  margins  of  the  sur- 
face are  the  mesial,  distal, 
occlusal,  and  cervical.  The 
surface  passes  so  gradu- 
ally into  the  mesial  and 
distal  that  it  is  somewhat 
difficult  to  define  these 
margins.  In  general,  the 
margins  converge  slightly 
in  the  direction  of  the 
root.  Both  the  occlusal  and  cervical  margins  are  similar  to  the 
corresponding  margins  of  the  buccal  surface. 

The  Mesial  Surface  of  the  Crown  (Fig.  92). — This  sur- 
face is  almost  an  unbroken  plane,  being  smooth  and  flat.  In 
some  instances  it  is  crossed  near  the  center  of  its  occlusal  margin 
by  a  continuation  of  the  mesial  groove,  but  this  is  seldom  so  pro- 
nounced as  to  divide  the  surface.  The  occlusal  third  of  the 
surface  is  inclined  to  a  slight  general  convexity,  providing  a 
point  of  contact  for  the  distal  surface  of  the  second  bicuspid, 
but  between  this  and  the  cervical  line  there  is  often  a  slight  con- 
cavity.    The  margins  of  the  surface  are  the  occlusal,   buccal. 


IlG.    92 


-Upper  First  Molak,  Mesial 
Surface. 


UPPER    FIRST    MOLAR. 


187 


Mesiobuc- 
cal  Root 

Distobuc- 
cal  Root 


lingual,  and  cervical.  The  first  named  is  formed  by  the  mesio- 
marginal  ridge  of  the  occlusal  surface,  and  is  concave  in  the 
direction  of  the  root.  The  buccal  and  lingual  margins  are 
rounded,  and,  unlike  the  lateral  margins  of  the  buccal  and 
lingual  surfaces,  diverge  in  the  direction  of  the  roots.  The 
cervical  margin  is  slightly  concave  in  the  direction  of  the  occlusal 
surface,  and  its  length  is  much  greater  than  that  of  any  other 
margin  of  the  crown.  This  surface  is  more  extensive  than 
either  the  buccal,  lingual,  or  distal,  and  is  about  equal  to  that  of 
the  occlusal.  When  the  fifth  cusp  is  present  it  alters  the  form 
of  the  lingual  margin  of  this  surface  by  crossing  it  near  the 
center,  and  extending  for 
some  little  distance  on  the 
face  of  the  surface. 

The  Distal  Surface  of 
the  Crown  (Fig.  93). — 
Taken  in  its  entirety,  this 
surface  usually  presents  a 
general  convexity.  The  lin- 
gual half  of  the  surface  is 
usually  somewhat  more  pro- 
minent than  the  buccal,  the 
latter  being  flattened  and 
frequently  slightly  concave, 
particularly  near  the  cervi- 
cal portion.  In  some  in- 
stances the  surface  is  trav- 
ersed by  a  continuation  of  the  distolingual  groove,  which,  after 
passing  over  the  distomarginal  ridge,  is  continued  in  a  longi- 
tudinal direction,  dividing  the  surface  into  two  equal  parts.  Not 
infrequently  this  groove,  instead  of  existing  as  such,  is  repre- 
sented as  a  shallow  depression,  often  extending  to  the  bifurcation 
of  the  roots.  The  margins  of  the  surface  are  four  in  number; 
the  occlusal,  which  closely  resembles  the  corresponding  margin 
of  the  mesial  surface  ;  the  buccal,  which  is  not  well  defined  ;  the 
lingual,  somewhat  angular ;  and  the  cervical,  formed  by  the 
cervical  line. 

The  Neck  of  the  Tooth. — When  looking  upon  the  buccal 


Lingual 
Root 

Disto- 

liMKual 
Cusp 

B 

Fig.  93. — Upper  First  Molar,  Distal 
Surface. 


i88  ANATOMY. 

surface  of  this  tooth,  the  constricted  portion  forming  the  neck 
is  greatest  at  a  point  immediately  above  the  cervical  line. 
Viewed  in  this  direction  the  crown  is  usually  bell-shaped,  and 
both  the  crown  and  the  base  of  the  roots  assist  in  producing 
the  neck.  Viewed  from  a  lingual  direction,  the  neck  is  a 
distinctive  feature,  but  is  seldom  so  marked  as  when  examined 
from  the  opposite  side.  When  studied  from  either  a  mesial  or 
a  distal  aspect,  the  neck  appears  above  the  cervical  line,  the 
prominent  fold  of  enamel  immediately  adjacent  to  this  line 
forcing  the  neck  rootward. 

The  Roots  of  the  Upper  First  Molar. — The  roots  of  this 
tooth  are  three  in  number,  two  of  which  are  on  the  buccal  side, 
and  are,  therefore,  called  mesiobuccal  and  distobuccal  roots,  and 
one  on  the  lingual  side,  known  as  the  lingual  root.  These  three 
roots  are  given  off  from  a  common  base,  which  is  sometimes 
referred  to  as  the  root,  while  those  parts  above  the  point  of 
trifurcation  are  considered  as  root-branches.  The  number, 
location,  and  form  of  the  roots  of  this  tooth  are,  perhaps,  more 
constant  than  those  found  in  connection  with  any  other  cuspidate 
tooth.  The  common  base  from  which  the  roots  are  given  off  is 
similar  in  contour  to  the  crown  of  the  tooth,  excepting  in  those 
cases  in  which  the  form  of  the  root  is  carried  over  this  base  to 
meet  the  neck  of  the  tooth. 

The  mesiobuccal  root  (Fig.  90)  is  flattened  from  mesial  to 
distal,  broad  at  its  base  from  buccal  to  lingual,  from  which  point 
it  gradually  tapers  to  the  apex.  At  the  base  the  mesiodistal 
measurement  is  less  than  one-third  that  of  the  buccolingual. 
In  its  course  it  is  first  inclined  to  the  mesial,  but  after  reaching 
the  center  of  its  length  it  makes  a  decided  distal  curve,  which 
looks  almost  directly  to  the  distal.  The  mesial  side  of  this  root 
is  decidedly  flattened  at  its  base,  but  as  the  center  of  the  surface 
is  reached  a  shallow  longitudinal  grobve  is  present,  which  is 
continued  to  the  region  of  the  apex.  The  distal  side  is  also 
possessed  of  a  similar  groove,  which  extends  throughout  its 
entire  length.  Both  the  buccal  and  lingual  sides  of  the  root  are 
smoothly  convex,  the  latter  being  only  about  half  the  width  ot 
the  former. 

The  distobuccal  root  (Fig.   90)   is   much  the  smallest  of  the 


UPPER   FIRST   MOLAR.  189 

three,  and,  while  inclined  to  flatness  on  its  mesial  and  distal  sides, 
it  is  much  more  rounded  than  the  mesial  root.  The  mesial  side 
is  provided  with  a  slight  longitudinal  groove,  and  in  rare 
instances  a  similar  groove  exists  on  the  distal  side.  The  buccal 
and  lingual  sides  are  similar  to  those  of  the  mesial  root.  The  root 
is  generally  straight,  and  tapers  gradually  from  base  to  apex, 
ending  in  a  rounded  point. 

The  lingtial root  (Fig.  91)  is  usually  the  largest  and  longest  of 
the  three,  and  is  more  rounded  in  form  than  either  of  the  buccal 
roots.  The  lingual  surface  is  inclined  to  flatness  near  its  base, 
and  is  provided  with  a  well-defined  longitudinal  groove,  which  is 
sometimes  independently  formed,  while  at  others  it  is  present  as 
a  continuation  of  the  lingual  groove.  This  root  being  the  only 
one  given  off  from  the  lingual  side  of  the  tooth,  is  constructed 
with  a  mesiodistal  measurement  about  equal  to  that  of  the  base 
of  the  crown  at  this  point.  From  its  place  of  beginning  it  passes 
first  in  a  lingual  and  then  in  a  buccal  direction,  forming  a  long 
curve  and  ending  in  a  sharp-pointed  apex. 

Bilious  Type. — The  upper  first  molar  of  this  temperament 
is  manifest  by  a  crown  with  angles  well  produced,  the  marginal 
ridges  and  cutting-edges  of  the  cusps  bold  and  well  marked. 
The  cusps  are  of  medium  length,  with  summits  angular  and 
pointed.  The  developmental  grooves  are  deep  and  often 
sulcate,  and  numerous  supplemental  grooves  are  found  upon 
the  occlusal  surface.  The  longitudinal  and  transverse  meas- 
urements of  the  crown  are  about  equal,  and  when  viewed  upon 
the  occlusal  surface,  the  angular  nature  of  its  anatomy  is  noted 
as  a  distinctive  feature.  The  neck  is  fairly  well  developed, 
giving  a  slight  bell-shape  to  the  crown.  The  cervical  line  is 
made  up  of  angles  rather  than  curves,  and  the  roots  are  long 
and  straight. 

Nervous  Type. — Like  the  teeth  previously  described  under 
this  class,  the  crown  of  this  tooth  is  of  greater  longitudinal  than 
transverse  extent  ;  the  neck  is  especially  well  formed,  producing 
a  decided  bell-shape  to  the  crown.  The  cusps  are  long  and 
penetrating,  the  marginal  ridges  sharply  defined,  as  are  also 
those  ridges  upon  the  central  incline  of  the  cusps.  The  grooves 
of  development  are  decided  and  frequently  sulcate.     The  buccal 


surface  is  rounded  and  smooth,  with  the  buccal  groove  extending 
well  toward  the  cervical  line.  The  lingual  surface  also  presents 
a  general  convexity,  and  is  usually  divided  by  the  lingual  groove. 
The  mesial  surface  is  convex  over  its  cervical  third,  and  the 
occlusal  margin  is  a  decided  convex  ridge,  serving  as  a  point  of 
contact  for  the  adjoining  tooth,  and  thus  forming  the  character- 
istic V-shape  common  to  this  temperament.  It  is  in  this  type 
that  the  fifth  cusp  is  most  frequently  present.  The  cervical  line 
is  much  curved,  and  the  roots  are  slim  and  frail. 

Sanguineous  Type. — The  crown  of  the  upper  first  molar 
of  this  type  usually  presents  a  slightly  greater  longitudinal  than 
transverse  extent.  The  angles  of  the  crown  are  poorly  formed, 
being  rounded  and  smooth.  The  cusps  are  of  moderate  length, 
and  are  rounded  in  their  nature ;  the  marginal  ridges,  as  well  as 
those  ridges  of  the  central  incline  of  the  cusps,  are  less  distinct 
than  either  of  the  forms  previously  described.  The  buccal  and 
lingual  surfaces  are  convex  and  seldom  broken  by  grooves ;  the 
mesial  and  distal  surfaces  are  convex  in  every  direction,  throw- 
ing the  point  of  contact  with  adjoining  teeth  near  the  center  of 
the  surface.  The  roots  are  inclined  to  be  large  and  oval  in  form, 
while  the  cervical  line  is  a  series  of  long  curves. 

Lymphatic  Type. — In  this  temperament  the  crown  is  much 
less  in  its  longitudinal  than  transverse  measurement.  The  neck 
of  the  tooth  is  poorly  defined,  the  crown  passing  into  the  root- 
base  without  a  marked  constriction.  The  mesial  and  distal 
surfaces  are  flattened  and  nearly  parallel  with  each  other, 
providing  a  broad  contact  surface.  The  buccal  and  lingual 
surfaces  each  present  a  marked  general  convexity,  the  latter 
being  frequently  broken  by  the  distolingual  groove.  The  tooth 
is  provided  with  cusps  which  are  short  and  heavy-set ;  the 
marginal  ridges,  as  well  as  all  the  ridges  common  to  the  occlusal 
surface,  are  poorly  defined.  The  developmental  grooves  are 
shallow  and  terminate  abruptly.  There  is  but  little  curvature 
to  the  cervical  line,  and  the  roots  are  short,  heavy-set,  and 
inclined  to  cluster  together. 


UPPER   SECOND    MOLAR. 


191 


UPPER  SECOND  MOLAR. 


7th  year    Sth  year    9th  year 


loth  year 
Fig.   94. 


Calcification  Begins  kkom  Four  Centers  About  the  Fifth  Year. 
Calcification  Completed,  Sixteenth  to  Eighteenth  Year. 
Erupted,  Twelfth  to  Fourteenth  Year. 
Average  Length  of  Crdwn,  .28. 

Average  Length  of  Root,  .51. 

Average  Length  Over  All,  .79. 

Calcification  in  this  tooth  takes  plact  in  precisely  the  same 
manner  as  that  of  the  first  molar,  but  the  formative  process  is 
much  later  in  beginning,  the  lime-salts  commencing  to  accumulate 
in  the  four  separate  lobes  about  the  fifth  year.  At  the  beginning 
of  the  si.xth  year  the  formation  of  the  cusps  is  completed,  soon 
after  which  they  coalesce  and  the  occlusal  surface  of  the  crown 
is  established.  At  the  beginning  of  the  eighth  year  fully  two- 
thirds  of  the  crown  is  calcified,  and  the  following  year  the  crown 
and  neck  are  completed  and  the  root-base  outlined.  By  the  tenth 
year  the  beginning  of  separate  root-development  is  observed ; 
at  the  twelfth  year,  or  at  the  time  of  eruption,  the  roots  are 
formed  to  about  one-half  of  their  completed  length,  the  process 
continuing  until  the  sixteenth  or  seventeenth  year,  when  calcifi- 
cation is  completed  and  the  apical  foramina  established  (Fig. 
94).  In  many  respects  this  tooth  closely  resembles  the  first 
molar  previously  described,  the  crown  presenting  the  same 
number  of  surfaces  similarly  named,  and  also  being  provided 
with  the  same  number  of  roots.  Notwithstanding  this  fact, 
there  are  a  number  of  ways  in  which  they  are  at  variance. 
The  crown   of  the  second    molar  is  smaller    than   that  of   the 


first,  and  the  quadrilateral  outline  common  to  the  first  molar 
is  much  compressed  and  broken  in  the  second.  The  distal 
cusps  are  much  smaller  proportionately  than  the  mesial  cusps, 
this  being  particularly  true  of  the  distoling-ual  cusp.  This 
reduction  in  size  of  the  distal  cusps  gives  to  that  portion  of  the 
occlusal  surface  a  slight  distal  incline. 

Occlusal  Surface  of  the  Crown  (Fig.  95). — The  general 
contour  of  the  crown  is  best  studied  by  viewing  it  directly  upon 
the  occlusal  surface  ;  this  aspect  also  shows  to  best  advantage 
the  difference  in  form  between  this  and  the  first  molar  tooth,  as 
shown  in  figure  87.  The  mesial  and  lingual  outlines  closely 
resemble  the  corresponding  outlines  on  the  first  molar,  but  the 


Buccal  Groove 


M< 
Tria 

^siobuccal 
ngular  Ridge 

Mesi 

al  Gro( 

Dve 

Mesi 

iomarg 

iiial  Ridge 

Mesi 

oliiigu; 

al  Cusp 

Distobuccal  Cusp 

Distal  Fossa 
Distolingual  Groove 


Central     Dislal  Incline  of 
Fossa       Mesiolingual  Cusp 

Fig.  95. — Upper  Second  Molar,  Occlusal  Surface. 


buccal  and  distal  are  much  at  variance,  the  former  passing  into 
the  latter  without  a  distinct  line  of  demarcation  existing  between 
the  two,  this  gradual  blending  of  one  into  the  other  being  at  the 
expense  of  the  distobucsal  angle  of  the  crown,  which  is  poorly 
developed.  The  crown  is  much  compressed  in  a  distobuccal- 
mesiolingual  direction,  making  this  measurement  of  the  occlusal 
surface  about  one-third  less  than  the  mesiobuccal-distolingual 
measurement.  The  cusps  are  much  inclined  to  cluster  toward 
the  center  of  the  surface,  this  being  especially  true  of  those  on 
the  lineual  half 


UPPER   SECOND   MOLAR.  193 

Maj'ginal  Ridges  of  the  Occlusal  Surface  (Fig.  94). — Like  the 
occlusal  surface  of  the  upper  first  molar,  this  surface  of  the 
second  molar  is  bounded  by  four  marginal  ridges — the  mesial, 
distal,  buccal,  and  lingual.  They  are  usually  less  marked  than 
those  found  on  the  first  molar,  and  are  much  more  variable 
in  their  individual  anatomy.  'Yht  mesiomarginal  ridge  extends 
from  the  summit  of  the  mesiobuccal  cusp  to  the  summit  of  the 
mesiolingual  cusp.  It  is  concave  in  the  direction  of  the  body  of 
the  crown,  and  is  broken  near  its  central  portion  by  the  mesial 
groove.  In  some  instances  one  or  more  small  supplemental 
grooves  are  found  to  cross  it.  Compared  with  the  mesiomar- 
ginal  ridge  of  the  first  molar,  its  length  is  much  less  and  the 
convexity  not  so  pronounced.  The  distomarginal  7'idge,  owing 
to  the  variation  in  form  and  size  of  the  distal  cusps,  is  difficult  to 
describe  definitely  ;  suffice  it  to  say  that  it  extends  from  the 
summit  of  the  distobuccal  to  the  summit  of  the  distolingual 
cusp.  The  concavity  is  V-shaped,  and  is  usually  crossed  near 
the  center  by  the  distolingual  groove.  In  some  instances  the 
distolingual  cusp  is  almost  wanting,  in  others  the  distobuccal  is 
but  litde  developed  ;  when  either  of  these  conditions  are  present, 
the  marginal  ridge  is  extended  either  to  the  buccal  or  to  the 
lingual,  in  a  measure  taking  the  place  of  the  missing  cusp. 
The  mesial  half  of  the  buccomarginal  ridge  closely  resembles 
the  corresponding  margin  of  the  first  molar ;  beginning  at 
the  mesiobuccal  angle  it  ascends  to  the  summit  of  the  mesio- 
buccal cusp,  after  which  it  descends  by  a  longer  incline  to 
the  buccal  groove.  The  distal  half  of  the  ridge,  unlike  that 
of  the  first  molar,  presents  much  variety,  its  form  being  con- 
trolled by  the  character  and  position  of  the  distobuccal  cusp; 
usually  small.  As  most  frequently  observed,  it  ascends  to  the 
summit  of  the  cusp,  and  in  so  doing  it  presents  a  decided 
lingual  inclination.  In  passing  down  the  distal  incline  the 
lingual  inclination  is  increased  and  gradually  passes  into  the 
distomarginal  ridge.  Branching  off  from  the  mesiomarginal 
ridge,  the  liiiguoniarginal  ridge  ascends  to  the  summit  of  the 
mesiolingual  cusp  and  descends  by  a  much  shorter  incline  to  the 
distolingual  groove.  This  portion  of  the  margin  is  thrown  well 
toward  the  center  of  the  surface,  the  location  of  the  cusp  carry- 
13 


194-  ANATOMY. 

ing  it  to  that  point.  Like  tlie  distal  half  of  the  buccal  margin, 
the  outline  of  the  distal  half  of  this  margin  is  controlled  by  the 
position  and  form  of  the  distolingual  cusp.  In  the  majority  of 
cases,  when  the  cusp  is  moderately  strong,  the  ascent  from 
the  distobuccal  groove  to  the  summit  of  the  cusp  is  short  and 
abrupt,  the  descent  being  somewhat  more  gradual,  and  with 
a  decided  buccal  inclination  it  passes  into  the  distomarginal 
ridge,  or  ends  abruptly  at  the  distal  end  of  the  distolingual 
groove. 

The  Cusps  and  Ridges  (Fig.  95). — This  tooth  is  provided 
with  four  lobes  or  cusps,  two  of  which  are  located  on  the  buccal, 
and  two  on  the  lingual  side.  They  are  usually  smaller  and  less 
angular  than  the  cusps  of  the  first  molar.  This  is  particularly 
true  of  both  the  distal  cusps,  and  especially  of  the  distolingual 
cusp,  which  is  often  quite  diminutive  and  occasionally  entirely 
wanting.  When  this  latter  condition  exists,  the  lingual  half  of 
the  surface  is  for  the  most  part  occupied  by  what  would  other- 
wise be  the  mesiolingual  cusp,  the  absence  of  the  distal  cusp 
permitting  the  distolingual  groove  to  occupy  a  position  near  the 
extreme  distolingual  angle,  that  portion  of  the  surface  which  is 
distal  to  the  groove  being  a  portion  of  the  distomarginal 
ridge. 

The  Mesiobuccal  Cusp  (Fig.  95). — Like  the  corresponding 
cusp  of  the  first  molar,  this  cusp  is  usually  the  longest  of  the 
four,  and  in  many  instances  covers  a  greater  extent  of  surface 
than  any  of  the  others.  Its  base  is  outlined  by  the  buccal  and 
mesial  grooves,  the  two  together  forming  the  mesiobuccal 
triangular  groove.  Descending  from  its  summit  to  the  buccal 
surface  is  the  mesiobuccal  ridge  ;  the  marginal  ridge  makes  a 
double  descent,  one  in  a  mesial  and  one  in  a  distal  direction, 
while  sloping  toward  the  central  fossa  is  the  mesiobuccal  tri- 
angular ridge.  The  cusp  is  seldom  traversed  by  supplemental 
grooves  such  as  are  found  on  the  corresponding  cusp  of  the 
first  molar. 

The  DistobiLccal  Cusp  (Fig.  95). — As  previously  stated,  this 
cusp  is  not  constant  in  its  form  ;  in  some  instances  it  is  bold  and 
well  produced,  corresponding  closely  to  the  mesiobuccal  cusp 
just  described.    When  thus  pronounced  it  is  possessed  of  ridges, 


UPPER    SECOND    MOLAR.  195 

and  bounded  by  grooves  which  are  similar  to  those  described  in 
connection  with  the  first  molar.  More  frequently  the  cusp  is 
much  rounded,  its  summit  being  carried  well  toward  the  center 
of  the  surface.  When  this  formation  exists,  the  buccal  ridge  is 
absent,  the  marginal  ridges  short  and  rounded ;  the  distobuccal 
triangular  ridge  which  descends  from  it  toward  the  center  of 
the  crown  is  short  and  heavy  set. 

The  Mesioliiignal  Cusp  (Fig.  95). — In  the  majority  of  instances 
this  is  the  largest  cusp,  particularly  when  there  is  a  degenerate 
tendency  in  the  distolingual  cusp.  Descending  from  its  summit 
are  a  number  of  ridges,  the  marginal  ridges  being  given  off  as 
already  described,  the  mesiolingual  triangular  ridge  descending 
the  central  incline  to  the  central  fossa,  and  when  the  cusp  has 
an  additional  mesiodistal  extent  by  the  presence  of  a  diminutive 
distal  cusp,  other  ridges  descend  in  the  same  direction.  The 
lingual  descent  of  the  cusp  is  smooth  and  more  rounded  than 
the  corresponding  surface  of  the  first  molar,  and  is  seldom 
elevated  in  the  form  of  a  definite  ridge.  The  central  outline  of 
this  cusp  is  marked  by  the  mesial,  the  distal,  and  the  distolingual 
grooves. 

The  Distolingual  Cusp  (Fig.  95). — In  no  other  cusp  do  we 
find  such  a  diversity  of  form  as  in  the  distolingual  cusp  of  the 
upper  second  molar.  In  some  instances  it  is  fully  as  promi- 
nent as  its  neighbor  just  described,  in  others  appearing  as  a 
mere  fold  of  enamel,  and  it  is  not  uncommon  to  find  it  entirely 
wanting,  the  distomarginal  ridge  extending  to  occupy  a  portion 
of  the  space  which  it  should  claim.  Deductions  might  be  drawn 
from  an  average  between  these  two  extremes,  wherein  the  e.\- 
isting  cusp  would  be  much  smaller  than  any  of  the  others,  the 
summit  rounded  rather  than  sharp,  but  with  a  decided  inclination 
to  occupy  the  extreme  distolingual  angle  of  the  surface,  in  this 
latter  respect  differing  from  the  distobuccal  cusp.  The  mesial 
and  buccal  outlines  of  the  cusp  are  formed  by  the  distolingual 
groove,  and  its  mesiolingual  incline  contributes  to  the  formation 
of  the  distal  fossa. 

The  Fossae  and  Grooves  of  the  Occlusal  Surface  (Fig. 
95). — These  in  name,  number,  and  general  form  are  similar  to 
those  of  the  first  molar.  The  central  fossa  is  never,  strictly  speak- 


196 


ing,  in  the  center  of  the  surface,  and  is  formed  by  tlie  central  in- 
cHne  of  the  mesiobuccal,  distobuccal,  and  mesioHngual  cusps.  It 
is  seldom  so  deep  as  the  central  fossa  of  the  first  molar.  The 
distal  fossa  is  more  or  less  pronounced,  its  size  and  position  being 
controlled  by  the  extent  of  development  in  the  distolingual  cusp. 
The  distolingual  groove,  which  usually  crosses  the  lingual  surface 
of  the  first  molar  near  its  center,  is  not  constant  in  its  location  on 
this  tooth,  in  some  cases  being  near  the  center,  in  others  near 
the  distolingual  angle  of  the  crown.  The  buccal  groove  is  never 
constant  in  its  location,  usually  crossing  the  buccomarginal 
ridge  and  passing  over  the  buccal  surface  near  its  mesiodistal 
center,  but  it  is  not  uncommon  to  find  it 
forced  to  the  distal  by  a  diminution  in  the 
size  of  the  distal  cusp. 

Buccal  Surface  of  the  Crown  (Fig. 
96). — The  most  constant  dilTerence  be- 
tween this  and  the  corresponding  surface 
of  the  upper  first  molar  is  the  wandering 
location  of  the  buccal  groove.  While  in 
the  majority  of  instances  it  may  be  found 
near  the  mesiodistal  center  of  the  surface, 
it  is  not  uncommon  to  find  it  passing  over 
the  distal  third,  or  even  as  far  posterior  as 
the  distobuccal  angle.  In  general,  the  sur- 
face is  somewhat  more  convex  and  neces- 
sarily less  extensive  than  the  buccal  surface 
The  buccal  ridges  which  descend  from  the 
summit  of  the  two  buccal  cusps  are  seldom  so  marked  as  those 
on  the  first  molar,  and  in  many  instances  the  distal  ridge  is 
wanting.  The  distal  half  of  the  surface  frequently  passes  into 
the  distal  surface,  by  a  long  gradual  sweep,  there  being  no  line 
of  demarcation  between  the  two. 

The  Lingual  Surface  of  the  Crown  (Fig.  97). — In  keeping 
with  the  other  surfaces  just  described,  the  lingual  surface  differs 
from  the  corresponding  surface  of  the  first  molar  in  that  it  pre- 
sents a  greater  general  convexity.  This  is  particularly  true  in 
passing  from  the  cervical  line  to  the  occlusal  surface.  The 
lineual  eroove  is  also  less  constant  in  its    location.     In  most 


Fig.  96.  —  Upper 
Second  Molar,  Buc- 
cal Surface. 


of  the  first  molar. 


UPPER   SECOND    MOLAR. 


197 


!  Ridge 


Fig.  97. — Upper  Second  Molar,  Lingual 
Surface. 


instances  it  is  to  be  found  a  little  to  the  distal  of  the  center,  in 
others  being-  as  far  posterior  as  the  e.xtreme  distal  third  of  the 
surface,  and  in  rare  instances  it  is  entirely  wanting.  The 
general  character  of  this 
surface,  which  is  smooth 
and  convex,  is  seldom 
broken  by  the  presence 
of  well-  defined  ridges, 
such  as  are  usually  found 
descending  from  the  lin- 
gual cusps  of  the  first 
molar.  As  previously  re- 
ferred to,  the  mesial,  dis- 
tal, and  buccal  surfaces,  as 
well  as  the  surface  under 
consideration,  are  propor- 
tionately smaller  than 
those  of  the  first  molar, 

and,  while  this  refers  to  both  the  transverse  and  longitudinal 
measurements,  it  is  particularly  applicable  to  the  latter. 

The  Mesial  Surface  of  the  Crown  (Fig.  98). — Aside  from 

this  surface  being  of  less 
extent  than  the  corres- 
ponding surface  of  the  first 
molar,  there  are  no  other 
differences  of  importance. 
In  many  instances,  how- 
ever, there  is  a  decided 
tendency  for  the  surface 
to  be  concave  from  buccal 
to  lingual,  the  convex  dis- 
tal surface  of  the  first 
molar  closely  fitting  into 
this  concavity.  Another 
variation  which  is  fre- 
quently observed  is  that 
of  the  longer  and  more  gradual  sweep  which  it  takes  in  passing 
into  the  lino-ual  surface. 


Fig.  98. — Upper  Second  Molar,  Mksiai, 
Surface. 


The  Distal  Surface  of  the  Crown  (Fig.  98). — This  differs 
from  the  distal  surface  of  the  first  molar  principally  in  its  more 
pronounced  convexity.  Its  general  form  is  also  much  influenced 
by  the  nature  of  the  two  distal  cusps.  If  one  or  the  other  of  these 
is  sparingly  developed,  either  the  buccal  or  lingual  half  of  the 
surface,  as  the  case  may  be,  is  quickly  rounded  off  to  pass  into 
the  deficient  lobe. 

The  Angles  of  the  Crown. — The  increased  inclination  for 
the  crown  of  this  tooth  to  general  convexity  dispels,  in  a  meas- 
ure, the  presence  of  angles,  as  such,  in  correspondence  with  the 

four  corners  of  the  first 
molar.  In  some  instances 
the  crown  is  represented 
as  a  fairly  well-formed  qua- 
drilateral, in  which  case 
the  angles  are  well  de- 
fined, but  usually  this 
outline  is  so  much  broken 
by  a  mesiodistal  compres- 
sion that  the  angular  form 
of  the  crown  is  entirely 
abolished.  But,  whatever 
the  form  of  the  crown 
may  be,  it  is  well  to  ad- 
here to  the  commonly 
accepted  term,  and  speak  of  that  point  at  which  the  sides  of  the 
crown  unite  as  the  angles,  each  being  named  in  accordance  with 
its  location. 

The  Neck  of  the  Tooth. — The  principal  variation  between 
the  neck  of  this  tooth  and  that  of  the  first  molar,  is  that  pro- 
duced by  the  greater  general  convexity  of  the  crown,  which 
contributes  to  the  production  of  a  neck  much  more  constricted. 
There  is  also  a  greater  variety  in  the  contour  of  the  neck,  inci- 
dent to  the  variation  in  the  general  outline  of  the  crown. 

The  Roots  of  the  Upper  Second  Molar. — These  are 
the  same  in  name  and  number  as  those  of  the  first  molar — 
two  buccal  and  one  lingual.  In  many  respects  they  differ 
from  the  roots  of  the  first  molar.     They  are  much  smaller,  fre- 


FiG.  99. — Upper  Second  Molar,  Distal 
Surface. 


UPPER   SECOND    MOLAR.  199 

quently  inclined  to  cluster  together,  and  are  often  fused,  in  some 
instances,  all  three  being  united,  in  others  the  union  existing 
between  but  two.  When  isolated,  each  root  usually  presents  a 
decided  distal  curve  near  its  apical  third.  When  the  crown  is 
flattened  from  mesial  to  distal,  as  before  described,  the  disto- 
buccal  root  is  forced  to  occupy  a  position  much  more  to  the 
lingual  than  that  assumed  by  the  mesiobuccal  root.  The 
lingual  groove  seldom  passes  over  the  lingual  root,  as  ob- 
served on  the  first  molar. 


UPPER  THIRD   MOLAR. 


m  0 


lolh  j-ear  nth  year  12th  year  uih  year  :Sth  year 

Fig.  100. 

Calcification  Begins,  Ninth  Year. 

Calcification  Completed,  Eighteenth  to  Twentieth  Year. 
Erupted,  Seventeenth  to  Twentieth  Year. 
Average  Length  of  Crown,  .24. 

Average  Length  of  Root,  .44. 

Average  Length  over  All,  .6S. 

Calcification  of  this  tooth  takes  place  in  precisely  the  same 
manner  as  that  in  the  first  and  second  molar,  with  the  excep- 
tion of  the  number  of  lobes,  which  are  sometimes  three  and 
sometimes  four.  The  lime-salts  begin  to  accumulate  between 
the  eighth  and  ninth  year,  and  continue  with  somewhat  more 
activity  than  that  of  the  first  or  second  molar.  Between 
the  ninth  and  tenth  year  the  three  or  four  cusps,  of  which  the 
future  tooth  is  to  be  composed,  have  coalesced,  and  by  the 
eleventh  year  calcification  in  the  crown  of  the  tooth  is  completed  ; 
at  the  end  of  the  following  year  the  roots,  which  are  variable  in 
number,  have  made  considerable  progress  ;  at  the  fourteenth 
year  they  are  calcified  to  about  half  their  length,  w^hile  at  a 
period  between  the  eighteenth  and  nineteenth  year  the  forma- 
tive process  is  completed  (Fig.  lOo).  This  tooth,  like  the  cuspid, 
is  usually  fully  formed  before  eruption  takes  place. 

This  tooth  is  subject  to  a  greater  variety  of  form  than  any 
other ;  in  rare  instances  it  is  similar  in  general  outline  and 
cusp  formation  to  the  first  molar,  but  in  a  vast  majority  of 
cases    it    is    dissimilar,   the    most  constant  deviation    being   its 


UPPER   THIRD   MOLAR.  201 

size,  which  on  the  average  is  about  one-third  less.  In  the 
accompanying  illustration  (Fig.  loi)  the  iorms  most  frequently 
met  with  are  shown.  It  will  be  observed  that  the  contour 
of  the  tooth  in  general  is  much  more  rounded  than  either 
the  first  or  second  molar.  The  buccal  angles  of  the  crown 
are  alone  well  marked,  the  mesial  and  distal  surfaces  passing 
into  the  lingual  surface  by  a  long,  gradual  sweep,  and  thus 
obliterating  the  lingual  angles.  In  the  vast  majority  of  instances 
the  tooth  is  tritubercular,  and  is  usually  made  so  by  the 
absence  or  diminutive  size  of   the  distolingual  cusp.     Just  as 


Fig.  ioi. — Various  Types  of  Upper  Third  Molar. 


this  cusp  was  inclined  to  degenerate  in  the  second  molar,  so 
we  find  this  retrograde  developmental  tendency  increased  in 
the  third  molar.  With  this  change  in  the  construction  of  the 
occlusal  surface,  there  is  a  corresponding  variation  in  the 
grooves,  ridges,  and  fossas. 

Mesial  Surface  of  the  Crown  (Fig.  102). — In  many  particu- 
lars this  surface  corresponds  in  form  and  outline  to  the  mesial 
surface  of  the  first  molar  ;  it  is,  however,  usually  much  more 
convex,  seldom  presenting  a  concavity  or  even  a  positive  flatness. 
The  surface  is  not  only  rounded  from  buccal  to  liuQ-ual,  but  also 


cal  Cusp 


Fig.   102. — Upper  Third  Molar,  Mesial 
Surface. 


from  the  cervical   line  to  its  occlusal  margin.     Thus  formed,  a 
point  of  contact  is  provided  near  the  center  of  the  surface.    The 

occlusal  margin,  the  buccal 
margin,  and  the  cervical 
margin  are  almost  identical 
to  those  of  the  first  molar, 
but  in  most  instances  the 
lingual  margin  is  vv^anting, 
the  surface  gradually  pass- 
ing into  the  lingual  without 
a  decided  line  of  demarca- 
tion. 

Distal  Surface  of  the 
Crown  (Fig.  103). — This 
surface  is  much  less  exten- 
sive in  comparison  to  the 
size. of  the  crown  than  the 
corresponding  surface  of  either  the  first  or  second  molars.  It  is 
decidedly  rounded  in  every  direction  and  is  frequently  crossed  by 
the  distal  developmental  groove,  and  sometimes  by  one  or  more 
supplemental  grooves.  The 
general  form  of  the  surface 
is  much  influenced  by  the 
presence  or  absence  of  the 
distolingual  cusp  ;  with  the 
former,  the  surface  is  more 
extensive,  presenting  less 
convexity  and  resembling 
more  closely  the  distal  sur- 
face of  the  first  and  second 
molars  ;  with  the  latter,  the 
extent  of  the  surface  is  de- 
creased and  the  convexity 
increased. 

Buccal  Surface  of  the  Crown  (F'ig.  104). — The  mesial  por- 
tion of  this  surface  is  in  no  way  at  variance  with  the  mesial  por- 
tion of  the  buccal  surface  of  the  first  or  second  molar,  but  much 
variety  of  form  exists  in  the  distal  portion.      The  buccal  groove 


Fig.  103.- 


-Upper  Third  Molar,  Distal 
Surface. 


UPPER   THIRD    MOLAR. 


Lingual  Root 


-Upper  Third  Molar, 
Surface. 


which  serves  to  separate  these  two  portions  is  located  well 
toward  the  distal  third  of  the  surface,  thus  reducing  the  size  of 
the  distal  portion  to  about  one-third  that  of  the  mesial  por- 
tion. In  general,  the  surface  is  but  little  more  convex  than 
the  corresponding  sur- 
face of  the  first  and  second 
molar.  Its  mesial  border 
is  definitely  outlined,  as 
are  also  the  cervical  and 
occlusal  margins,  but  the 
distal  margin  can  not  be 
definitely  located,  the  sur- 
face tending  to  pass  grad- 
ually into  the  distal  sur- 
face. Like  the  distal,  the 
extent  of  this  surface  is 
much  regulated  by  the 
size  and  shape  of  the  dis- 

tobuccal  cusp,  which,  like  the  distolingual  cusp,  is  inclined  to  de- 
generate. 

Lingual  Surface  of  the  Crown  (Fig.  105). — Like  the  distal 
surface  previously  described,  the  form  of 
this  surface  is  much  influenced  by  the  pres- 
ence or  absence  of  the  distolingual  cusp. 
When  this  cusp  is  wanting  or  but  little  de- 
veloped, the  surface  presented  is  decidedly 
convex  and  smooth  ;  in  many  instances  the 
mesiodistal  curvature  described  is  almost  a 
perfect  semicircle,  and  in  passing  from  the 
cervical  line  to  the  occlusal  margin  the  sur- 
face is  carried  well  toward  the  center  of 
the  crown  by  a  long  gradual  sweep  toward 
the  lingual.  The  lingual  groove  is  usually 
absent.  The  change  in  form  produced  by 
the  presence  of  the  distolingual  cusp  is 
principally  noticeable  in  a  less  pronounced  convexity  and  the 
presence  of  the  lingual  groove,  which  may  be  noticed  as  a  slight 
depression  or  as  a  well-defined  groove.     This  groove,  when  pres- 


FiG.  105. — Upper 
Third  Molar,  Lingual 
Surface. 


ent,  is  always  located  near  what  would  represent  the  distolingual 
angle  of  the  crown  ;  the  distolingual  cusp  seldom  if  ever  being  of 
sufficient  size  to  force  its  location  near  the  center  of  the  surface 
as  in  the  first  molar.  In  some  instances  this  groove  is  shown 
upon  the  lingual  surface  when  the  cusp  is  not  present  ;  in  this 
case  the  distomarginal  ridge  represents  in  a  manner  the  cusp 
by  its  bold,  heavy  development. 

Occlusal  Surface  of  the  Crown  (Fig.  io6). — When  looking 
directly  upon  this  surface,  an  opportunity  is  presented  to  study 
the  general  contour  of  the  crown  ;  the  most  noticeable  difference 
in  this  respect  between  this  tooth  and  the  first  molar  being  ob- 
served   in    its    smaller    size,  and    the  absence  of  well-marked 

angles.  It  will  be  noted 
that  the  mesial  and  buc- 
cal outlines  in  a  meas- 
ure resemble  the  corres- 
ponding outlines  of  the 
first  and  second  molars, 
but  there  is  scarcely  any 
similarity  existing  when 
comparing  the  distal  and 
lingual  outlines.  In  some 
instances  the  crown  is 
triangular  (Fig.  loi);  in 
others  the  mesiobuccal 
and  buccomarginal  outlines  form  an  obtuse  angle,  the  free  ends 
of  which  are  joined  together  by  a  long  semicircle,  the  latter 
constituting  the  distal  and  lingual  outlines  (Fig.  loi).  Again, 
almost  the  reverse  of  this  last-mentioned  form  is  seen,  the 
buccal  and  distal  outlines  constructing  the  angle,  while  the  semi- 
circular connection  between  the  two  is  made  up  of  the  mesial 
and  lingual  outlines. 

The  Marginal  Ridges. — The  viesioinarginal  ridge  is  usu- 
ally well  defined,  and  in  most  instances  is  crossed  near  its 
center  by  the  mesial  groove,  and  frequently  by  two  or  more 
supplemental  grooves.  This  marginal  ridge  is  probably  the 
most  constant  in  form,  the  numerous  variations  to  which  the 
surface  is  liable  seldom  making  any  material  alteration    in  it. 


FiF.   io6. — Upper  Third  Molar,  Occlusal 
Surface. 


UPPER   THIRD    MOLAR.  205 

Unlike  the  ridge  above  described,  the  dislomaroiual  ridge 
is  most  variable  in  its  construction,  nearly  all  of  the  forms 
characteristic  of  the  occlusal  surface  exerting  a  controlling- 
influence  over  it.  In  rare  instances  the  ridge  resembles  that 
of  the  first  and  second  molars,  but  this  form  is  most  frequently 
interfered  with  by  the  absence  or  diminutive  size  of  the  disto- 
lingual  cusp,  the  ridge  itself  frequently  supplying  the  place  of 
the  cusp.  In  many  cases  the  ridge  is  elevated  near  its  central 
part  by  being  reinforced  by  a  portion  of  the  oblique  ridge. 
When  the  distolingual  cusp  is  wanting,  this  ridge  not  infre- 
quently descends  from  the  summit  of  the  distobuccal  cusp  to 
the  distal  groove  and  from  this  point  ascends  obliquely  to  the 
summit  of  the  lingual  cusp.  The  bucco- 
viarginal  ridge  may  be  described  as 
similar  in  most  respects  to  the  correspond- 
ing margin  on  the  first  and  second  molars, 
the  principal  variation  being  in  the  distal 
half,  which  is  much  shorter  and  less  pro- 
nounced. In  the  linguomarginal  ridge, 
again,  much  variety  in  outline  is  noticeable. 
In  nearly  all  instances  the  ridee  is  thrown         ^  ,,       ^ 

-'  "  Fig.  107. — Upper  Third 

much  nearer  the  center  of  the  body  of  the      Molar,    Occlusal    sur- 

,        ,  ,  ....  •  1    •  FACE,  WITH  Distolingual 

crown,  and,  when    the  tooth  is  bicuspid    in        Cus?   and    Distal   Fossa 

form,  it  simply  makes  a  mesial  ascent  of      ^"''^''^  ^^^^^^^. 
the  lingual  cusp,  followed   by  a  gradual 

incline,  and  passes  into  the  distal  ridge,  as  above  noted.  When 
the  distolingual  cusp  is  present,  the  ridge  is  similar  to  that 
upon  the  first  and  second  molars,  with  the  exception  of  the 
distal  portion,  which  is  less  clearly  marked. 

The  Cusps  (Fig.  io6). — As  previously  stated,  the  form 
most  frequently  met  with  is  tritubercular,  two  of  the  cusps 
being  upon  the  buccal  and  one  upon  the  lingual  half  of  the 
surface. 

The  Jllesiobuccal  Cusp  (Fig.  106). — This  cusp  corresponds  in 
nearly  every  particular  to  the  mesiobuccal  cusp  of  the  first  and 
second  molars  ;  it  is  the  most  constant  in  size  and  form  of  the 
three.     Its  summit  is  usually  angular,  and  the  numerous  ridges 


which  descend  from  it  are  well  defined  and  similar  in  name 
and  number  to  those  of  the  first  molar. 

Distobjucal  Cusp  (Fig.  io6). — The  constant  inclination  to 
degeneracy  in  the  distal  portion  of  the  crown  of  the  tooth  is 
noticeable  in  this  cusp,  which  is  much  smaller  than  the  mesio- 
buccal  and  scarcely  half  as  large  as  the  corresponding  cusp  of 
the  first  and  second  molars.  In  some  instances,  however,  it  is 
inferior  only  in  size,  retaining  its  angularity,  being  possessed 
of  small  but  well-defined  ridges. 

The  Lingual  Cusp  (Fig.  io6). — When  the  three  cusps  alone 
are  present,  this  one  is  much  the  largest,  the  extent  of  the  sur- 
face covered  being  all  of  the  lingual  half  of  the  crown.  The 
summit  of  the  cusp,  which  is  thrown  well  toward  the  center  of 
the  body  of  the  crown,  is  prominent,  but  seldom  angular.  Only 
in  rare  instances  will  there  be  found  a  lingual  ridge  descending 
therefrom,  but  the  central  incline  is  usually  marked  by  a  number 
of  wrinkles  or  folds  of  enamel  resembling  minute  ridges.  The 
central  boundary  of  this  cusp  Is  marked  by  the  mesial  and  distal 
developmental  grooves. 

The  Distolingiial  Cusp  (Fig.  io6). — It  Is  the  presence  or 
absence  of  this  cusp  that  contributes  most  to  the  variations 
present  In  the  crown.  When  present,  It  is  usually  diminutive  in 
size,  and  Is  without  definite  form.  In  many  Instances  nature 
is  apparently  attempting  to  cast  it  off  in  precisely  the  same 
manner  In  which  she  is  attempting  to  add  to  the  first  molar  by  a 
development  of  the  "fifth  cusp,"  the  distobuccal  cusp  appearing 
to  hang  to  the  distollngual  angle  of  the  crown  in  a  manner 
very  similar  to  the  "  fifth  cusp."  When  thus  situated.  It  Is  sepa- 
rated from  the  body  of  the  crown  by  a  groove,  which  cannot  be 
considered  as  being  upon  the  occlusal  surface.  When  located 
In  Its  normal  position,  it  has  for  its  Inner  boundary  the  disto- 
llngual groove. 

The  Fossae  and  Grooves  of  the  Occlusal  Surface. — The 
great  variety  andy^rw  common  to  this  surface  exerts  a  con- 
trolling Influence  over  the  size,  number,  and  position  of  the 
grooves  and  fossae.  In  the  tritubercular  class  the  central  fossa 
alone  is  present.  The  developmental  grooves,  wdth  the  excep- 
tion of  the  buccal,  are  not  definitely  outlined,  but,  descending 


UI'PER   THIRD   MOLAR.  207 

toward  the  fossae  from  the  central  incHne,  are  numerous  small 
ridges  divided  from  each  other  by  a  like  number  of  diminutive 
supplemental  grooves.  The  distal  groove  is  sometimes  well 
defined,  and  crosses  over  the  oblique  ridge,  which  in  this  type 
becomes  the  distomarginal  ridge.  When  the  distolingual  cusp 
is  present,  all  of  the  ridges  and  grooves  are  more  pronounced. 
In  this  case  the  central  fossa  corresponds  more  closely  to  the 
central  fossa  of  the  other  molar  teeth,  this  resemblance  increas- 
ing just  in  proportion  as  the  size  of  the  distolingual  cusp  in- 
creases. The  distal  fossa,  in  a  vast  majority  of  instances,  is 
present  as  a  mere  pit  ;  the  size  of  this  fossa  is  likewise  much 
controlled  by  the  extent  of  development  in  the  distolingual  cusp. 
Where  the  distomarginal  ridge  is  supplementary  to  the  disto- 
lingual cusp,  the  distolingual  groove  lies  between  the  former  and 
the  oblique  ridge.  Another  peculiarity  found  only  upon  the  occlu- 
sal surface  of  this  tooth  is,  what  appears  to  be  an  effort  upon  the 
part  of  the  cusps  to  cluster  toward  the  center.  This  is  common 
only  to  those  teeth  possessing  three  cusps,  and  accompanying  this 
form  the  central  fossa  shows  a  number  of  fantastically  arranged 
grooves  and  ridges  which  ascend  the  cusps,  passing  over  the 
marginal  ridges  and  breaking  them  into  a  number  of  small 
tubercles. 

Temperamental  Types. — The  third  molar  tooth  is  probably 
less  influenced  by  the  character  and  habits  of  the  individual  than 
any  other  tooth  in  the  mouth.  The  inclination  to  a  general  de- 
generacy is  no  doubt  favored  by  civilization.  With  a  constant 
decline  in  the  functional  activity,  brought  about  by  the  pres- 
ent culinary  methods  common  to  civilization,  this  tooth  in  a 
measure  becomes  useless,  and  nature  is  gradually  making  an 
effort  to  cast  it  off  While  there  are  undoubtedly  many  indi- 
viduals possessed  of  the  highest  mental  attainments  with  the 
third  molar  as  fully  developed  as  either  the  first  or  the  second, 
this  condition  is  usually  confined  to  those  possessed  of  little  in- 
tellectuality. If  in  general,  the  temperament  of  the  subject  be 
taken  into  consideration,  the  cusp-formation  on  this  tooth  will 
correspond  in  a  relative  degree  to  that  on  the  bicuspids  and 
molars. 


CHAPTER    IX. 

A  DESCRIPTION  OF  THE  LOWER  TEETH  IN  DETAIL.-CALCIFICA- 
TION,  ERUPTION,  AND  AVERAGE  MEASUREMENTS.-THEIR  SUR- 
FACES, RIDGES,  FOSS/C,  GROOVES,  SULCI,   ETC. 

THE   LOWER  TEETH. 

In  most  respects  the  lower  teeth  correspond  to  the  upper, 
but  in  each  class  we  find  a  slight  variation  existing  between 
the  two  sets.  As  compared  to  the  upper  incisors,  the  crowns 
of  the  lower  incisors  are  more  slender  and  somewhat  mare 
angular  in  outline.  The  roots  are  more  slender,  proportionately 
longer,  more  flattened  laterally,  and  seldom  crooked.  The 
crowns  of  the  lower  incisors  are  probably  more  constant  in 
form  than  those  of  any  other  teeth  in  the  mouth,  seldom  vary- 
ing except  in  size.  The  mesiodistal  measurement  of  the  crown 
of  the  lateral  incisor  is  a  trifle  greater  than  that  of  the  central, 
a  condition  exactly  the  reverse  to  that  of  the  upper  incisors. 
The  labial  and  the  lingual  surfaces  of  these  teeth  are  smooth, 
and,  with  the  exception  of  young  teeth,  show  but  little  trace  of 
the  developmental  process  by  the  presence  of  grooves,  fissures, 
etc.  The  outline  of  the  lower  cuspids  is  almost  identical  to 
that  of  the  corresponding  teeth  in  the  superior  arch,  excepting 
that  they  are  in  every  way  more  slender.  The  bicuspids  are 
proportionately  smaller  in  every  direction,  have  their  cusps 
much  less  developed  than  their  upper  antagonists,  and  are 
seldom  found  with  more  than  one  root.  The  crowns  of  the 
lower  molars  are  somewhat  larger  than  those  of  the  upper, 
and  are  provided  with  five  cusps  instead  of  four,*  and  they  are 
attached  to  the  alveolus  with  two,  instead  of  three,  roots.  In  the 
incisors,  cuspids,  and  bicuspids  the  process  of  development  is 
the  same  as  in  the  corresponding  upper  teeth,  calcification 
taking  place  from  the  same  number  of  centers  along  the  coronal 

*The  lower  firat  mo'ar  his  five  cusps  in  ninety  per  cent,  of  cases,  while,  in  the  second, 
five  cusps  are  present  in  about  fifty  per  cent. 

208 


LOWER   CENTRAL   INCLSOR.  209 

extremities.  In  the  molars,  however,  development  may  proceed 
from  five  centers  instead  of  four,  as  in  the  upper  molars.  The 
manner  of  development,  and  the  period  at  which  this  action 
takes  place,  so  nearly  corresponds  with  that  of  the  upper 
teeth  that  the  process  will  not  be  repeated. 


LOWER  CENTRAL  INCISOR. 

Calcification  Begins,  First  Year  after  Birth. 

Calcification  Completed,  about  the  Tenth  Year. 
Erupted,  Seventh  to  Eighth  Year. 
Average  Length  of  Crown,  .34. 

Average  Length  of  Root,  .47. 

Average  Length  Over  All,  .81. 

Like  the  upper  central  incisor,  this  tooth  presents  for  ex- 
amination four  surfaces,  a  cutting-edge,  and  various  angles, 
margins,  etc.  By  the  union  of  the  labial  and  lingual  surfaces 
at  the  cutting-edge  the  incisive  feature  is  established  and  the 
double  incline  plane  common  to  in- 
cisors produced. 

The  Labial  Surface  of  the 
Crown  (Fig.  io8). — This  surface  is 
smooth  and  convex,  its  general  out- 
line resembling  an  inverted  cone,  the 
base  of  which  is  formed  by  the  cut- 
ting-edge and  the  apex  by  the  cervical 
line.  The  margins  of  the  surface  are, 
with  the  exception  of  the  cutting-edge, 
not  so  well  defined  as  those  of  the 
upper  central.  Near  the  cutting- 
edge  the  mesial  and  distal  margins 
pass  somewhat  abruptly  into  the  re- 
spective lateral  surfaces,  but  as  the 
neck  of  the  tooth  is  approached,  they 
are  much  rounded.  The  incisive  mar- 
gin is  squarely  cut,  and  is  nearly  at  right  angles  with  the  long  axis 
of  the  tooth.  The  cervical  margin  is  fairly  well  defined,  and  is 
deeply  concave  in   the  direction   of  the  root.      Except  in   very 

H 


Fig.  108. — Lower  Incisor,  Right 
Side,  Labial  Surface. 


young  teeth,  this  surface  is  seldom  much  broken  by  the  labial 
grooves  ;  but  in  certain  types  one  or  more  transverse  ridges 
may  be  found  occupying  the  cervical  third.  The  mesiodistal 
diameter  at  the  cutting-edge  is  about  one-third  greater  than  at 
the  cervical  line,  and,  while  these  measurements  are  likely  to 
vary  in  accordance  with  the  temjoerament  of  the  subject,  this 
variation  is  not  so  pronounced  as  in  the  upper  incisor.  The 
surface  is  frequently  inclined  to  flatness  near  the  incisive 
margin,  the  general  convexity  becoming  more  marked  as  the 
cervical  line  is  approached. 

The  Lingual  Surface  of  the  Crown  (Fig.  109). — In  general 
outline  this  surface  resembles  the  labial,  with  the  exception  of 

the  cervical  margin,  the  lines  of 
Lingual  Grooves  whicli  are  somewhat  more  acute. 

The  surface  presents  a  marked 
concavity  from  the  cutting-edge 
to  the  cervical  ridge,  and  also  a 
slight  transverse  concavity  near 
the  incisive  margin.  All  of  the 
margins  are  more  definite  than 
those  of  the  labial  surface.  The 
mesial  and  distal  margins  are 
formed  by  the  marginal  ridges 
common  to  these  borders,  but 
these  ridges  are  not  so  well  de- 
fined as  those  of  the  upper 
incisors.  The  cervicomarginal 
ridge  is  present  as  a  well-rounded  band  of  enamel,  but  is  never 
a  well-defined  cingulum,  or  cuspule.  The  depression  between 
these  marginal  ridges  is  so  slight  that  it  can  scarcely  be  referred 
to  as  a  fossa,  although  usually  characterized  as  the  lingual  fossa. 
The  lingual  grooves  are  generally  more  pronounced  than  the 
corresponding  developmental  grooves  of  the  labial  surface,  but 
end  more  or  less  abruptly  before  reaching  the  cervical  ridge. 
The  mesiodistal  measurements  of  the  surface  are  a  trifle  less 
than  the  corresponding  measurements  of  the  labial  surface. 

The  Mesial  Surface  of  the  Crown  (Fig.   no). — The  out- 
line   of   this    surface   is    exactly  the   reverse  of   the  labial  and 


Fig.  109. — Lower  Incisor,  Right 
Side,  Lingual  Surface,  Strongly 
Developed. 


LOWER   CENTRAL   INCISOR. 


no. — Lower 
Incisor,  Mesial  Sur- 
face. 


lingual  just  described,  being  a  cone,  with  its  base  directed 
downward  or  in  the  direction  of  the  root,  while  its  apex  is 
formed  by  the  mesial  extremity  of  the  cutting-edge.  The  cer- 
vical margin  of  the  surface,  or  that 
represented  by  the  base  of  the  cone, 
is  concave ;  the  labial  and  lingual 
margins  are  rounded  over  the  cer- 
vical third  and  inclined  to  angularity 
near  the  cutting-edge.  There  is  a 
slight  convexity  over  the  entire  sur- 
face, which  is  most  marked  near  the 
center.  The  lingual  half  of  the  cer- 
vical portion  slopes  away  to  the  dis- 
tal, passing  gradually  into  the  lingual 
surface.  By  the  union  of  this  surface 
with  the  cutting-edge  and  the  labial 
and  lingual  surfaces,  the  mesial  angle 
of  the  crown  is  formed.  This  angle 
is  well  outlined  and  reaches  out  to- 
ward the  median  line,  giving  to  this  portion  of  the  crown  a 
prominent  appearance.  Near  the  cutting-edge  the  surface  pre- 
sents a  slightly  rounded  promi- 
nence, which  provides  a  point  of 
contact  with  the  corresponding 
tooth  of  the  opposite  side. 

The  Distal  Surface  of  the 
Crown  (Fig.  iii). — In  a  general 
way  this  surface  closely  resembles 
that  of  the  opposite  or  mesial  side 
of  the  crown.  Near  the  cutting- 
edge  the  surface  is  usually  more 
prominent  and  presents  a  more 
marked  convexity,  and  near  the 
cervical  margin  it  is  flattened  and 
sometimes  slightly  concave.  The 
union  of  this  surface  with  the  cut- 
ting-edge and  the  labial  and  lingual  surfaces  forms  the  distal  angle 
of  the  crown,  which,  like  the  mesial  angle,  is  square  and  well 


-Lower  Incisor,  Distal 
Surface. 


defined.     The  margins  of  the  surface  are  in  no  way  different 
from  those  of  the  mesial  surface. 

The  Cutting-edge. — In  the  young  tooth  this  incisive  margin 
is  thin  and  generally  divided  into  three  distinct  parts  (Fig.  112) 
by  the  developmental  grooves,  but  these  disappear  so  early  that 
they  can  scarcely  be  considered  in  connection  with  a  description 
of  the  fully  developed  tooth ;  in  fact,  the  cutting-edge  of  this, 
as  well  as  that  of  all  the  lower  incisors,  is  so  susceptible  to 
change  by  mechanical  abrasion  that  a  normal  condition  is  of 
but  short  duration.  After  the  disappearance  of  the  primitive 
cusps,  and  before  further  abrasion  has  taken 
Developmental  Grooves      placc,   the  edgc  IS   fairly  sharp  and  placed 

B  nearly  at  right  angles  with  the  crown.  As 
in  the  upper  incisors,  the  cutting-edge  is  in 
a  line  with  the  long  axis  of  the  tooth.  The 
labial  margin  of  the  edge  is  slightly  convex, 
while  the  lingual  is  irregularly  concave  to 
the  same  extent. 
The  Cervical  Margin. — This  marginal 
line,  which  is  marked  by  the  extent  of  the 
enamel  cap,  corresponds  closely  to  that  of  the 
upper  incisors,  dipping  down  with  a  grace- 
ful concavity  on  the  labial  and  lingual  sur- 
faces, with  a  corresponding  convexity  on  the 
mesial  and  distal  surfaces.  The  prominence 
of  this  enamel  margin,  together  with  the 
nature  of  the  curvature,  is  much  influenced 
by  the  tooth  type. 
The  Neck  of  the  Tooth. — A  distinctive  feature  of  this 
tooth  is  found  in  the  convergence  of  its  mesial  and  distal  sur- 
faces in  passing  from  the  cutting-edge  rootward,  thus  produc- 
ing a  neck  much  constricted  from  mesial  to  distal.  When  ex- 
amined from  either  the  mesial  or  distal  surface,  this  feature  is 
scarcely  noted,  the  crown  passing  into  the  root  with  little  more 
than  the  cervical  line  as  a  mark  of  separation.  The  labial  and 
lingual  portions  of  the  neck  are  rounded  and  narrow,  while  the 
two  lateral  sides  are  flat  and  broad. 

The  Root. — The  root  of  this  tooth  is  usually  smaller  than 


Fig.  112.  —  Young 
Lower  Incisor,  with 
Cutting  -  edge,  show- 
ing THE  Lines  of  De- 
velopment. 


LOWER    LATERAL   INCISOR.  213 

that  of  any  other  tooth  in  the  mouth.  It  is  much  flattened  from 
mesial  to  distal,  while  the  labial  and  lingual  aspects  are  rounded 
and  narrow.  Besides  being  flattened  and  broad,  the  mesial  and 
distal  sides  are  usually  found  with  a  longitudinal  depression  ex- 
tending from  a  point  near  the  base  of  the  root  almost  to  its 
apex.  These  surfaces  gradually  taper  from  the  base  to  the 
apex,  while  the  labial  and  lingual  first  widen  from  the  base  and 
then  gradually  taper  to  the  apex.  The  contour  of  the  root-base 
is  generally  reduced  at  the  apical  extremity,  although  in  some 
instances  the  latter  is  a  rounded  point.  While  the  root  of  the 
tooth  is  usually  straight,  there  is  sometimes  a  tendency  for  the 
apical  third  to  have  a  slight  distal  inclination. 


LOWER  LATERAL  INCISOR. 

Calcification  Begins,  First  Year  after  Birth. 

Calcification  Completed,  Tenth  to  Eleventh  Year. 
Erupts,  Eighth  to  Ninth  Year. 
Average  Length  of  Crown,  .35. 

Average  Length  of  Root,  .50. 

Average  Length  over  All,  .85. 

The  crown  of  this  tooth  differs  from  the  central  incisor  in 
being  broader  from  mesial  to  distal  at  the  cutting-edge,  resulting 
in  a  crown  more  strongly  bell-shaped.  The  cutting-edge,  instead 
of  being  at  right  angles  to  the  long  axis  of  the  tooth,  slopes 
to  the  distal  at  the  expense  of  the  distal  angle,  which  is  much 
rounded,  while  the  mesial  angle  closely  resembles  the  corre- 
sponding angle  of  the  central  incisors.  The  labial  and  mesial 
surfaces  do  not  differ  materially  from  the  corresponding  surfaces 
of  the  central  incisor,  excepting  that  the  lingual  more  frequently 
shows  the  lines  of  development,  and  the  distal  is  at  variance  in 
having  that  portion  which  contributes  to  the  formation  of  the 
distal  angle  extended  and  prominent.  The  marginal  ridges  of 
the  lingual  surface  are  probably  more  definitely  outlined  than 
those  of  the  central  incisor,  and  the  crown  in  general  presents  a 
stronger  appearance.  The  neck  of  the  tooth  is  similar  to  the 
neck  of  the  central  incisor,  as  is  also  the  root,  with  the  excep- 
tion of  a  slio'ht  addition  to  its  lenoth. 


LOWER  CUSPID. 


calcifrcation  begins,  third  year  after  birth. 

Calcification  Completed,  Twelfth  to  Thirteenth  Year. 
Erupts,  Twelfth  to  Thirteenth  Year. 
Average  Length  of  Crown,  .40. 

Average  Length  of  Root,  .60. 

Average  Length  over  All,  i.oo. 

There  is  probably  a  greater  similarity  existing  between  the 
upper  and  lower  cuspid  teeth  than  in  any  other  class  ot 
teeth  in  the  mouth.  Occupying  as  they  do  a  prominent 
position  in  the  dental  arch,  and  being  called  upon  to  perform 

the  double  function   of  incis- 
Labiai  Grooves  |p,g  ^pj  teaHug  the  food,  their 

crowns  are  strong  and  heavy- 
set,  and  their  roots  long  and 
firmly  anchored  in  the  alveoli. 
Like  the  upper  cuspid,  the 
crown  of  the  lower  is  sur- 
mounted by  a  single  cusp, 
from  the  summit  of  which  de- 
scend a  mesial  and  a  distal 
cutting-edge.  There  is  also  a 
labial,  lingual,  mesial,  and  dis- 
tal surface  presented  for  ex- 
amination. 

The  Labial  Surface  of 
the  Crown  (Fig.  113). — The 
crown  of  the  tooth,  being  a 
little  longer  than  that  of  the  upper  cuspid,  gives  to  this 
surface  the  appearance  of  being  more  slender,  when  in 
reality  there  is  but  little  difference  in  the  width  of  the  two 
teeth.  This  surface  is  smooth  and  convex,  and,  while  the  labial 
grooves  are  usually  present,  they  are  not  so  marked  as  those 
found  upon  the  corresponding  upper  tooth.  A  pronounced 
feature  of  the  surface  is  the  labial  ridge,  which  extends  from  the 
summit  of  the  cusp  to  the  cervical  line,  providing  additional 
strength  to  the  crown.     Aside  from  this  ridge  and  the  labial 


Fig.  113. 


-Lower    Cuspid, 
face. 


Ladial   Sur- 


LOWER   CUSPID. 


215 


Lingual  Grooves 


grooves,  the  surface  is  occasionally  broken  by  one  or  more 
transverse  ridges  over  the  cervical  portion.  The  margins  of  the 
surface  closely  resemble  those  of  the  upper  cuspid,  the  incisive 
and  mesial  being  definite  in  character,  while  the  distal  is  made 
equally  indefinite  by  the  passing  of  the  labial  into  the  distal 
surface  by  a  gentle  curve. 

The  Lingual  Surface  of  the  Crown  (Fig.  114). — The 
ridges  and  grooves  of  this  surface  are  far  less  bold  in  their 
character  than  those  of  the  upper  cuspid.  The  lingual  ridge, 
which  divides  the  surface  into  two  equal  parts,  extends  from  the 
summit  of  the  cusp  to  the  base  of  the  cervical  ridge,  while  the 
marginal  ridges  pass  root- 
ward  from  the  angles  of  the 
crown,  and,  uniting,  form  the 
cervical  ridge.  The  slight 
depressions  between  the  lin- 
gual ridge  and  the  marginal 
ridges  correspond  to  the  pal- 
atal grooves  of  the  upper 
cuspid,  but  in  this  tooth  par- 
take more  of  the  nature  of 
fossae. 

The  Mesial  Surface  of 
the  Crown  (Fig.  115). — A 
peculiarity  found  in  connec- 
tion with  this  surface  is  the 
general  plane  existing  be- 
tween the  crown  and  root- 
surface.  In  all  other  teeth  the  mesial  and  distal  surfaces  are 
found  to  bulge  somewhat  beyond  the  corresponding  surface  of 
the  root,  but  this  surface  of  the  lower  cuspid  is  not  only  usually 
in  a  direct  line  with  the  mesial  surface  of  the  root,  but  is  occa- 
sionally inclined  to  the  distal,  resulting  in  a  crooked  or  bent 
appearance  to  the  tooth.  In  addition  to  this  individual  pecu- 
liarity, the  surface  is  flat  and  passes  by  a  long  curve  to  meet  the 
lingual  surface. 

The  Distal  Surface  of  the  Crown  (Fig.  1 1 6). — This  surface 
is  somewhat  less  in  extent  than  the  mesial  surface,  and,  in  place 


114. — Lower  Cuspid,  Lingual 
Surface. 


Cervica 
Ridge 


-Lower  Cuspid,  Mesial 
Surface. 


of  being  flat  and  in  line  with  the  root-surface,  that  portion  near 
the  angle  of  the  crown  presents  a  marked  convexity,  while  that 

near  the  cervical  line  is  fre- 
quently slightly  concave.  This 
general  form  of  the  surface  fur- 
ther assists  in  producing  the 
distal  crook  previously  referred 
to.  The  lingual  margin  is  well 
defined  and  somewhat  angu- 
lar, while  the  surface  passes 
so  gradually  into  the  labial 
that  a  positive  line  of  demarca- 
tion can  scarcely  be  said  to 
exist. 

The  Cusp  and  Cutting- 
edges. — In  most  respects  these 
are  similar  to  the  correspond- 
ing parts  of  the  upper  cuspid. 
The  length  of  the  mesial  cutting-edge  is  usually  somewhat  less 
than  that  of  the  distal,  but  this  difference  is  seldom  so  marked  as 
that  found  in  the  upper  cuspid.  The  mesial  and  distal  angles  of 
the  crown  are  equally  as  pro- 
nounced as  those  of  the  cor- 
responding upper  tooth. 

The  Neck  of  the  Tooth. 
— This  is  shown  by  a  fairly 
well-marked  constriction,  but 
the  passing  of  the  mesial 
surface  of  the  crown  into  the 
mesial  surface  of  the  root  is 
not  broken  by  this  circular 
depression.  On  account  of 
this  latter  feature  the  neck 
of  this 'tooth  is  somewhat  less 
pronounced  than  that  of  the 
upper. 

The  Root  of  the  Lower  Cuspid. — The  root  of  this  tooth  is 
somewhat  shorter  and  more  flattened  on  its  mesial  and  distal 


-Lower  Cuspid,  Distal 
Surface. 


THE   LOWER   BICUSPIDS.  217 

sides  than  that  of  the  upper  cuspid,  this  lateral  flatness  fre- 
quently amounting  to  a  decided  longitudinal  depression  or 
groove.  As  referred  to  in  the  description  of  the  crown,  the 
mesial  side  of  the  root  is  continuous  in  a  direct  line  with  this 
surface  of  the  crown,  but  as  the  apical  third  of  the  root  is 
approached,  there  is  frequently  found  a  slight  distal  inclination 
which  affects  alike  both  the  mesial  and  distal  sides.  The  labial 
and  lingual  surfaces  of  the  root  are  abruptly  convex  and  taper 
very  gradually  from  the  cervical  line  to  the  apex,  while  the 
mesial  and  distal  surfaces  taper  much  more  rapidly,  the  four 
ending  in  a  slender  apex  usually  flattened  from  mesial  to  distal. 


THE  LOWER  BICUSPIDS. 

In  many  respects  these  teeth  are  similar  to  the  bicuspids  of 
the  upper  jaw,  but  there  are  a  few  minor  points  of  dissimilarity. 
They  are  somewhat  shorter  and  smaller  in  every  respect ; 
their  crowns  are  much  more  rounded  and  the  cusps  are  never 
so  strongly  developed.  Unlike  the  upper  bicuspids,  the  buccal 
and  lingual  cusps  are  connected  by  a  transverse  ridge.  The 
roots  are  much  less  flattened  from  mesial  to  distal,  and  are 
seldom  bifurcated. 

LOWER  FIRST  BICUSPID. 

Calcification  Begins,  about  the  Fourth  Year. 

Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupted,  Tenth  to  Eleventh  Year. 
Average  Length  of  Crown,  .30. 

Average  Length  of  Root,  .54. 

Average  Length  over  All,  .84. 

In  general,  the  crown  of  this  tooth  is  much  more  rounded  and 
smaller  in  all  its  measurements  than  that  of  the  upper  bicus- 
pid. The  buccal  surface  presents  a  much  greater  convexity, 
which  results  in  forcing  the  summit  of  the  buccal  cusp  well 
toward  the  center  of  the  long  axis  of  the  tooth.  The  mesio- 
distal  and  buccolingual  measurements  of  the  crown  are  nearly 
equal,  and  about  correspond  to  the  maximum  length  ot  the 
crown.  As  in  the  upper  bicuspids,  the  development  ot  this 
tooth  is  similar  to  that  of  the  incisors  and  cuspids,  the  buccal 


cusp  being  derived  from  three  lobes,  while  the  lingual  results 
from  a  single  center. 

The  Occlusal  Surface  of  the  Crown  (Fig.  117). — This 
surface  is  so  unlike  that  of  the  corresponding  surface  of  the 
upper  first  bicuspid  that  a  separate  description  without  further 
comparative  reference  is  required.  In  general  outline  the  form 
of  a  rounded  triangle  is  approached,  the  buccal  margin  serving 
as  one  side  of  the  triangle,  while,  by  the  union  of  the  mesial  and 
distal  margins  to  form  the  lingual,  the  remaining  sides  are 
established. 

The  Buccal  Qisp. — rAs  previously  stated,  the  summit  of  this 
cusp  is  thrown  well  toward  the  center  of  the  surface.  Descend- 
ing from  it  are  four  well-defined  ridges — the  buccal  ridge  to  the 
buccal  surface,  the  mesial  cutting-edge,  the  distal  cutting-edge, 

and  the  triangular  ridge,  the 
Summit  of  Buccal  Cusp  lattcT  descending  in  a  lingual 

direction  to  meet  the  lingual 


^^         _^_J  Buccal  Groove     rido;e    or   cusp.     This  ridge 

uccal  Ridge  ^^S  -^HM  ^j     ■   i  p-,  *  ^  ^ 

^^  ^^    ""^    '  divides  the  surface  into  two 

parts,    the    center   of    each 
being    marked    by   a   well- 
defined  pit — the  mesial  and 
i^ni^ua.  iwuK^.  distal  pits.     The  mesial  and 

Fig.  117. — Lower  First  Bicuspid,  ,.        ,  . 

Occlusal  Surface.  distal  cuttmg-edges  are  fre- 

quently crossed  by  the  buccal 
grooves,  and  mark  the  line  of  union  between  the  central  and  two 
lateral  lobes  of  the  buccal  cusp.  The  marginal  ridges,  one  of  which 
begins  at  the  mesial  angle  and  the  other  at  the  distal  angle, 
pass  to  the  lingual,  where  they  unite  to  form  the  lingual  ridge 
or  cu,sp. 

The  Lingual  Cusp. — This  cusp  is  seldom  well  developed,  and 
corresponds  to  the  cervical  ridge  of  the  incisors  and  cuspids. 
The  extent  of  development  in  the  lobe  is  extremely  variable, 
in  some  instances  amounting  to  little  more  than  a  continuation 
of  the  mesio-  and  disto-marginal  ridges,  while  in  others  there  is 
a  building-up  of  the  enamel  in  the  form  of  a  small  tubercle. 
When  this  latter  condition  is  present,  the  triangular  ridge  of  the 
buccal  cusp  contributes  to  its  formation.     The  triangular  ridge 


THE   LOWER    BICUSPIDS. 


Fig.  iiS. — Right  Lower  First  Bi- 
cuspid, Buccal  Surface. 


frequently  divides  into  two  or  more  smaller  ridges,  which  usually 

end  in  the  mesial  pit,  but  in  some  instances  they  continue  to  the 

lingual,    and    divide    the    linofual 

ridge   into  two  or  more  smaller 

tubercles. 

The  Buccal  Surface  of  the 

Crown  (Fig.  ii8). — This  surface 

is  smooth  and  convex  in  all  direc- 
tions, and  in  general  outline  there 

is  but  little  variation  between  it 

and  the  corresponding  surface  of 

the    upper    first  bicuspid.     It    is 

traversed  from  the  point  of  the 

cusp  to  the  cervical  line  with  a 

rounded  ridge,  the  buccal  ridge, 

upon  either  side  of  which  are  the 

buccal  grooves. 

Lingual  Surface  of  the  Crown  (Fig.  119). — This  surface 

is   more  or  less  extensive  in  accordance  with  the  character  of 

the  lingual  lobe.  In  most  instances 
the  measurement  from  the  summit 
of  the  cusp  or  ridge  to  the  cervical 
line  is  about  one-half  that  of  the 
same  measurements  on  the  buccal 
surface.  From  mesial  to  distal  a 
well-rounded  convexity  is  present, 
while  from  the  occlusal  margin  to 
the  cervical  line  it  is  straight  or 
only  slightly  convex.  The  surface 
passes  so  gradually  into  the  mesial 
and  distal  surfaces  that  no  definite 
lateral  margins  exist. 

The   Mesial  Surface  of  the 
Crown  (Fig.  120). — In  the  region 
of  the  occlusal  margin  this  surface 
is  prominent,  with  a  marked  con- 
vexity from  buccal   to  lingual  ;    but   as  the   cervical   margin  is 

approached,  the  surface  recedes  to  the  distal,  and  is  flattened 


al  Cusp  Triangular 
Ridge 


Fig.   119. — Left  Lower  First  Bi- 
cuspid, Lingual  Surface. 


Fig.   120. — Left  Lower  First  Bicus- 
pid, Mesial  Sukface. 


or  is  possessed  of  a  slight  general  convexity.     The  occlusal  and 
cervical  margins  alone  are  well  defined,  the  buccal  being  grace- 
fully rounded,  while  the  surface 
passes  to  the  Hngual  with  a  long 
curve. 

The  Distal  Surface  of  the 
Crown  (Fig.  1 21). — There  is  but 
little  difference  between  this  and 
the  mesial  surface  ;  the  occlusal 
portion  of  the  surface  is  some- 
what less  prominent,  resulting  in 
less  of  the  bell-shaped  appear- 
ance to  this  side  of  the  crown. 

The  Neck  of  the  Tooth.— 
The  neck  of  this  tooth  is  marked 
by  a   well-defined    constriction, 
the   enamel  of  the  crown  sud- 
denly  folding   in   to    meet    the 
cementum  of  the  root  at  the  cervical  line,  forming  a  band  or 
ridge  which   completely  encircles   the  tooth.     The  amount  of 
constriction  appears  to  be  evenly 

distributed  between  the  various  Triangular  Ridge 

parts,  so  that,  viewed  in  all  direc- 
tions, the  neck  becomes  a  dis- 
tinctive feature  of  the  tooth. 

The  Root  of  the  Lower 
First  Bicuspid. — The  root  of 
this  tooth  is  usually  straight  and 
tapers  gradually  from  base  to 
apex.  In  rare  instances  it  is  bi- 
furcated, and  when  thus  formed, 
those  portions  beyond  the  point 
of  separation  are  more  or  less 
crooked.  In  the  single  root  the 
apical  third  often  curves  slightly 
to  the   distal.     The  buccal  and 

lingual  sides  are  convex  throughout  their  entire  length,  while  the 
mesial  and  distal  may  be  slighdy  convex,  flattened,  or  provided 


Lingual  Ridge 


121.  —  Right   Lower   First    Bi- 
cuspid, Distal  Surface. 


THE    LOWER   BICUSPIDS. 


with  a  slight  longitudinal  concavity.  In  passing  from  buccal  to 
lingual  the  mesial  and  distal  sides  converge,  thus  resulting  in  a 
narrowinor  of  the  lingual  side  of  the  root. 


LOWER  SECOND  BICUSPID. 

Calcification  Begins,  Between  the  Fourth  and  Fifth  Year. 
Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupts,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .31. 

Average  Length  of  Root,  .56. 

AvER.'iGE  Length  over  All,  .87. 

In  general  contour  this  tooth  is  similar  to  the  lower  first 
bicuspid,  excepting  that  the  crown  is  somewhat  more  rounded 
and  the  lingual  cusp  more  fully  developed,  this  latter  feature 
causing  it  to  closely  resemble  the  upper  bicuspids.  The 
crown  is  frequently  a  trifle 
shorter  than  that  of  the  lower 
first  bicuspid,  but  the  length 
of  the  root  generally  exceeds 
that  of  the  latter,  making  this 
the  longer  tooth  of  the  two. 

The  Occlusal  Surface  of 
the  Crown  (Fig.  122). — The 
occlusal  surface  of  this  tooth  pre- 
sents a  greater  variety  in  form 
than  any  other  tooth_of  its  class. 
The  general  outline  of  the  sur- 
face is  that  of  a  broken  circle,  in  most  instances  the  mesial 
and  distal  margins  showing  almost  as  much  of  a  convexity  as 
that  of  the  buccal  and  lingual.  The  summit  of  the  buccal  cusp 
usually  extends  well  toward  the  center  of  the  surface,  but  it  is 
sometimes  forced  toward  the  buccal  by  an  increased  develop- 
ment in  the  lingual  cusp.  The  buccal  grooves,  which  cross  the 
mesial  and  distal  cutting-edges  of  the  buccal  cusp,  are  seldom 
so  well  defined  as  those  of  the  first  bicuspid,  but  they  occasion- 
ally pass  over  these  marginal  ridges  and  form  well-marked 
grooves,  which  end  in  the  mesial  and  distal  pits.  The  triangular 
ridge  of  the  buccal  cusp  is  usually  more  prominent  than  in  the 


122. — Lower  Second  Bicuspid, 
Occlusal  Surface. 


ANATOMY. 


first  bicuspid,  and  divides  the  surface  into  two  portions,  which 
are  about  equal  in  extent,  the  center  of  each  portion  being  pro- 
vided with  a  small  pit — the  mesial  and  distal  pits.  As  in  the  first 
bicuspid,  the  mesio-  and  disto-marginal  ridges  begin  at  the 
mesial  and  distal  angles  of  the  crown,  pass  to  the  lingual,  and, 
uniting,  form  the  lingual  ridge  or  cusp.  The  lingual  cusp,  while 
generally  well  developed,  is  never  so  prominent  as  the  buccal. 
The  lingual  lobe  is  sometimes  divided  by  a  groove  which  passes 
from  buccal  to  lingual,  thus  forming  three  cusps  upon  the  sur- 
face.    When  this  latter  condition  is  present,  the  mesial  and  distal 

grooves  are  fully  outlined  from  the 
Buccal  Ridge  mesio-  and  disto-marginal  ridges 

to  the  center  of  the  surface,  where 
they  unite  with  the  groove  previ- 
ously referred  to  and  form  a  cen- 
tral pit  or  fossa.  Another  form 
frequently  met  with  is  one  in  which 
the  surface  closely  resembles  that 
of  the  upper  bicuspids,  two  well- 
defined  cusps  being  present,  sepa- 
rated from  each  other  by  a  central 
groove,  which  passes  from  mesial 
to  distal  and  joins  the  triangular 
grooves  at  these  points.  The  re- 
semblance to  the  upper  bicus- 
pids is  further  increased  by  the 
presence  of  two  small  pits,  one  on  the  mesial  and  one  on  the 
distal  half  of  the  surface. 

The  Buccal  Surface  of  the  Crown  (Fig.  123). — The 
principal  variation  between  this  and  the  buccal  surface  of  the 
lower  first  bicuspid  is  that  it  is  less  extensive  and  the  buccal 
grooves  somewhat  less  defined.  It  presents  a  general  con- 
vexity, which  is  most  pronounced  near  the  center,  between 
which  point  and  the  occlusal  margins  it  is  slightly  inclined  to 
flatness.  The  summit  of  the  buccal  cusp  is  usually  to  the 
mesial  of  the  center  of  the  occlusal  margin,  so  that  the  mesial 
cutting-edge  is  considerably  longer  than  the  distal,  this  fact  also 
resultine  in  forcing  the  buccal  ridg^e  to  the  mesial  of  the  center 


123. — Right   Lower   Second 
CUSPID,  Buccal  Surface. 


THE   LOWER    BICUSPIDS. 


124 — Lower  Second  Bicuspid, 
Lingual  Surface. 


of  the  surface.  The  mesial  angle  of  the  crown,  as  observed 
when  looking-  directly  upon  the  buccal  surface,  is  in  a  direct  line 
with  the  mesial  side  of  the  root, 

..   .^  1  1*  1  1  1  Buccal  Grooves 

while  the  distal  angle  extends 
beyond  this  corresponding  line, 
and  gives  a  prominent  or  bulging 
appearance  to  this  section  of  the 
crown. 

The  Lingual  Surface  of  the 
Crown  (Fig.  124). — Proportion- 
ately, this  surface  is  more  exten- 
sive than  the  corresponding  sur- 
face of  the  first  bicuspid,  this 
increase  being  produced  by  the 
additional  development  of  the 
lingual  cusp.  It  is  well  rounded 
from  mesial  to  distal,  and  passes 
into    these    surfaces  without  the 

existence  of  a  positive  line  of  separation.  From  the  cervical 
line  to  the  occlusal  margin  a  slight  convexity  is  present.     The 

general  outline  of  the  surface 
is  much  influenced  by  the  con- 
ditions present  upon  the  oc- 
clusal surface. 

The  Mesial  Surface  of 
the  Crown  (Fig.  125). — In 
the  region  of  the  occlusal 
margins  this  surface  is  decid- 
edly convex  from  buccal  to 
lingual,  but  in  passing  toward 
the  cervical  line  a  gradual 
flatness  is  apparent,  which, 
however,  seldom  amounts  to 
a  perfect  plane.  While  there 
is  a  gradual  convergence  of 
this  and  the  distal  surface  to- 
ward the  root,  it  is  not  so  marked  as  that  of  the  first  bicuspid, 
resulting  in  less  of  the  bell-shaped  appearance  to  the  crown. 


Triangular  Ridge 

Lingual  Cusp   I 

B^IBB 

Mesioniar- 

|^P!^B 

ginal  Ridge 

■  <^M^B 

1  r  fl 

1     ^1 

Buccal  Cusp 
liuccal  Ridge 


125. — Lower  Second  Bicuspid, 
Mesial  Surface. 


The  Distal  Surface  of  the  Crown  (Fig.  126). — Tlie  descrip- 
tion given  of  the  mesial  surface  applies  equally  well  to  this,  there 
being  but  slight  variation  existing  between  the  two.  Occasion- 
ally this  surface  will  present  a  greater  convexity  in  the  region 

of  the  occlusal  margin,  but  this  is 
not  a  constant  feature. 

The  Neck  of  the  Tooth.— 
The  crown  of  the  tooth  being  some- 
what smaller,  and  the  root  propor- 
tionately larger  and  longer  than 
that  of  the  first  bicuspid,  results  in 
diminishing  the  amount  of  constric- 
tion at  the  neck,  and  for  that  reason 
this  feature  is  less  definite. 

The  Root  of  the  Tooth.— As 
previously  stated,  the  root  of  this 
tooth  is  larger  and  longer  than  that 
of  the  first  bicuspid.  The  mesial 
and  distal  sides  are  flattened  and 
frequently  provided  with  a  longitudinal  groove.  In  some  in- 
stances it  is  rather  blunt,  ending  in  a  heavy,  rounded  apex  ;  in 
others  it  tapers  very  gradually  from  the  base  to  the  apex,  ending 
in  a  sHm,  pointed  extremity. 


Fig.  126. — Lower  Second  Bicuspid, 
Distal  Surface. 


THE  LOWER  MOLARS. 


THE  LOWER  FIRST  MOLAR. 


Calcification  Begins,  about  One  Month  Before  Birth. 
Calcification  Completed,  Ninth  to  Tenth  Year. 
Erupts,  Sixth  to  Seventh  Year. 
Average  Length  of  Crown,  .30. 

Average  Length  of  Root,  .52, 

Average  Length  over  All,  .82. 

The  process  of  development  in  this  tooth  corresponds  to 
that  of  the  upper  first  molar,  calcification  beginning  upon 
the  coronal  extremities  as  early  as  the  eighth  fetal  month,  the 
crown  being  completely  calcified  by  the  fifth  year,  the  roots 
formed  and  the  apical  foramina  established  by  the  eleventh  year. 


THE    LOWER    MOLARS. 


Buccal 
Cusp 


There  is,  however,  one  important  difference  between  the  devel- 
opment of  this  tooth  and  the  corresponding  upper  tooth : 
that  of  calcification  taking  place  usually  irom  Jive  centers  instead 
of  four,  and,  as  a  result,  we  find  the  occlusal  surface  provided 
with  five  well-developed  cusps  separated  from  one  another  by 
five  developmental  grooves.  When  compared  with  the  upper 
first  molar,  the  crown  of  this  tooth  is  found  to  be  somewhat  less 
in  size  ;  in  general  outline  it  is  subject  to  a  greater  variation 
and  is  much  more  angular  in  its  nature.  The  mediodistal 
measurement  of  the  crown  is  nearly  always  greater  than  the 
buccolingual,  and  the  length  of  the  crown  from  the  occlusal 
margins  to  the  cervical  line  is  proportionately  less  than  that  of 
the  corresponding  upper  molar. 

The  Occlusal  Surface  of  the  Crown  (Fig.  127). — The 
general  outline  of  the  crown 
is  best  studied  when  looking 
directly  upon  this  surface. 
Two  principal  varieties  exist : 
one  in  which  the  sides  or  mar- 
gins of  the  surface  appear  to 
be  flattened  or  straightened 
out,  and  the  other  when  these 
same  margins  are  gracefully 
rounded.  In  either  form  the 
buccal  line  is  the  longest,  so 
that  the  mesial  and  distal  lines 
converge  to  meet  the  lingual. 

This  common  form  gives  to  the  buccal  angles  an  acute  character, 
while  the  lingual  angles  are  about  equally  obtuse.  The  surface 
is  divided  into  five  distinct  or  developmental  portions,  each  of 
which  is  surmounted  by  a  cusp,  named,  as  their  location  indi- 
cates, mesiobuccal,  buccal,  distobuccal,  mesiolingual,  and  disto- 
lingual.  Separating  these  parts  are  five  developmental  grooves 
— the  mesial,  the  distal,  the  buccal,  the  lingual,  and  the  disto- 
buccal. The  four  former  cross  the  marginal  ridges  from  the 
various  surfaces  and  end  in  the  central  fossa,  while  the  latter 
passes  from  the  distobuccal  angle  and  joins  the  distal  groove, 
their  union  being  marked  by  a  slight  depression  or  pit — the 
IS 


Fig.  127.- 


-Lower  First   Molar,  Occlusal 
Surface. 


distal  pit.  Brandling  off  from  the  various  grooves  are  a  number 
of  supplemental  grooves,  the  presence  of  which  results  in  the 
production  of  a  number  of  smaller  ridges. 

The  Marginal  Ridges  of  the  Occlusal  Stirface. — Properly 
speaking,  these  are  only  two  in  number,  the  mesiomarginal 
ridge  and  the  distomarginal  ridge.  Those  margins  which  cor- 
respond to  the  buccal  and  lingual  ridges  of  the  upper  molars 
are  so  broken  by  the  various  cusps  and  developmental  grooves 
that  a  definite  marginal  ridge  scarcely  exists,  as  will  be  observed 
by  the  description  of  these  parts. 

The  mesiomarginal  ridge  is  strongly  outlined,  passing  from 
the  mesiobuccal  to  the  mesiolingual  angle  of  the  crown  in  the 
form  of  a  bold  angular  ridge.  In  some  instances  it  is  broken 
near  the  center  by  the  mesial  groove  passing  over  it  to  reach  the 
mesial  surface,  in  others  being  further  divided  by  numerous  small 
supplemental  grooves. 

The  distomarginal  ridge  is  much  shorter  and  less  decided 
than  the  mesial,  and  extends  from  the  distobuccal  to  the  disto- 
lingual  angle.  In  nearly  every  instance  it  is  broken  by  the 
distal  groove,  which  crosses  it  to  reach  the  distal  surface. 

The  buccomarginal  ridge  is  formed  by  the  various  ridges 
which  descend  in  a  mesial  or  distal  direction  from  the  three 
buccal  cusps.  Near  the  center  the  margin  is  broken  by  the 
buccal  groove,  and  it  is  again  broken  at  its  distal  third  by  the  dis- 
tobuccal groove,  both  of  which  pass  over  it  to  reach  the  buccal 
surface  of  the  crown.  This  is  much  the  longest  margin  of  the 
surface,  and  in  its  entirety  presents  a  gradual  buccal  convexity. 

The  linguomarginal  ridge  is  principally  made  up  of  the 
distal  incline  from  the  mesial  cusp,  and  by  the  mesial  incline 
from  the  distal  cusp.  Near  the  center  it  is  broken  by  the  lin- 
gual groove,  which  passes  over  it  to  reach  the  lingual  surface. 
This  margin,  unlike  the  buccal,  is  not  always  convex,  but  in 
many  instances  is  almost  a  straight  line,  extending  from  the 
mesial  to  the  distal  angle. 

The  Cusps  (Fig.  127). —  The  Mesiobnccal  Cusp  (Fig.  127). — 
This  is  usually  the  largest,  though  not  always  the  longest,  cusp  ot 
the  group.  It  is  bounded  by  the  mesial  and  buccal  surfaces  and 
by  the  mesial  and  buccal  grooves,  which  together  form  the  mesio- 


THE    LOWER    MOLARS.  227 

buccal  triangular  groove.  Descending  from  the  summit  of  this 
cusp  to  the  distal  is  a  well-defined  ridge, — a  part  of  the  bucco- 
marginal  ridge, — while  in  a  mesial  and  lingual  direction  the  de- 
scending ridges  contribute  to  both  the  bucco-  and  mesio-mar- 
ginal  ridges.  Descending  toward  the  center  of  the  surface  and 
ending  in  the  central  fossa  is  the  mesiobuccal  triangular  ridge. 

The  Buccal  Cusp  (Fig.  127). — This  cusp,  which  is  placed  a 
little  to  the  distal  of  the  center  of  the  buccal  surface,  is  sepa- 
rated from  the  mesiobuccal  cusp  by  the  buccal  groove,  and 
from  the  distobuccal  cusp  by  the  distobuccal  groove.  It  is 
about  one-half  the  size  of  the  mesiobuccal  cusp,  and  a  trifle  less 
in  length.  Descending  from  it  are  two  ridges,  one  in  a  mesial 
and  one  in  a  distal  direction,  which  form  a  portion  of  the  bucco- 
marginal  ridge  ;  descending  to  the  buccal  surface  is  the  buccal 
ridge,  while  the  central  incline  gives  place  to  a  fourth  ridge — 
the  buccotriangular  ridge. 

The  Distobuccal  Cusp  (Fig.  127). — This  cusp  is  much  the 
smallest  of  the  five,  and  is  located  at  the  distobuccal  portion  of 
the  crown,  in  some  instances  being  nearest  the  buccal  surface, 
in  others  forced  to  the  distal  by  an  increase  in  the  size  of  the 
buccal  cusp.  It  is  separated  from  the  buccal  cusp  by  the  disto- 
buccal groove,  and  from  the  distolingual  cusp  by  the  distal 
groove.  The  ridges  which  descend  from  it  contribute  to  both 
the  bucco-  and  disto-marginal  ridges,  and  descending  toward 
the  distal  pit  is  the  distobuccal  triangular  ridge. 

The  Mesiolingual  Cusp  (Fig.  127). — This  cusp  is  second  in 
size,  and  frequently  the  longest  and  most  pointed.  It  has  for 
its  boundaries  the  mesial  and  lingual  surfaces,  and  the  mesial 
and  lingual  grooves.  The  ridge  which  descends  from  it  in  a 
mesiobuccal  direction  assists  in  forming  the  mesiomarginal 
ridge,  while  that  which  passes  to  the  distal  forms  a  part  of  the 
linguomarginal  ridge.  In  the  direction  of  the  central  fossa  a 
pronounced  ridge  is  present, — the  mesiolingual  triangular  ridge, 
— which  is  often  supplemented  by  one  or  more  smaller  ridges 
running  in  the  same  direction. 

The  Distolingual  Cusp  (Fig.  127). — This  cusp  usually  occu- 
pies the  distolingual  portion  of  the  crown,  although  sometimes 
being  forced  well  toward  the  lingual  by  the  distobuccal  cusp. 


ANATOMY. 


It  is  separated  from  the  mesiolingual  cusp  by  the  hngual  groove, 
and  from  the  distobuccal  by  the  distal  groove.  Two  of  the 
ridges  which  descend  from  it  assist  in  forming  the  Hnguo-  and 
disto-marginal  ridges,  while  the  one  which  descends  the  central 
incline  is  the  distolingual  triangular  ridge.  The  central  incline 
of  the  mesiobuccal,  buccal,  mesiolingual,  and  distolingual  cusps 
contribute  to  the  formation  of  the  central  fossa,  while  the  buccal, 
distobuccal,  and  distolingual  central  inclines  assist  in  forming 
the  distal  pit  or  fossa. 

The  Buccal  Surface  of  the  Crown  (Fig.  128). — This  is 
the  most  extensive  of  the  lateral  surfaces  of  the  crown.     It  is 

convex   from   mesial  to 
Buccal  Groove  distal,  and  also  from  the 

occlusal  margin  to  the 
cervical  line.  The  width 
of  the  crown  from  the 
mesial  to  the  distal  an- 
gle is  always  somewhat 
greater  than  that  at  the 
cervical  line,  the  differ- 
ence being  governed 
by  the  typal  form  of  the 
tooth.  A  litde  to  the 
mesial  of  the  center  of 
the  surface  is  the  buc- 
cal groove,  which,  after 
crossing  the  buccomar- 
ginal  ridge,  is  usually 
quite  deep  ;  but  as  it  proceeds  in  the  direction  of  the  root  it  grad- 
ually disappears,  or  it  may  end  abruptly  in  a  well-defined  pit 
— the  buccal  pit.  The  distobuccal  groove  enters  the  surface  near 
the  distobuccal  angle,  and  gradually  becomes  less  pronounced  as 
it  passes  rootward.  It  is  seldom  so  well  defined  as  the  buccal 
groove,  and  usually  ends  when  about  half-way  to  the  cervical 
line.  The  occlusal  margin  is  made  irregular  by  the  presence  of 
the  three  buccal  cusps  ;  the  cervical  margin  is  nearly  straight 
from  mesial  to  distal,  and  is  surmounted  throughout  by  a  strong 


-Lower  First  Molar,  Buccal 
Surface. 


THE   LOWER   MOLARS. 


Distolingual    Mesiolingual 
Cusp  Cusp 


Fig.  129. — Lower  First  Molar, 
Lingual  Surface. 


enamel    fold,  the    cervicobuccal    ridge.     The  mesial  margin  is 
longer  than  the  distal,  but  neither  of  them  are  well  defined. 

The  Lingual  Surface  of  the  Crown  (Fig.  129). — This  sur- 
face is  smooth  and  convex  in  every 
direction.  It  is  generally  divided  into 
two  portions,  a  mesial  and  a  distal, 
which  are  nearly  equal  in  extent. 
This  separation  is  formed  by  the  lin- 
gual groove,  which  is  sometimes  deep 
and  sulcate,  at  others  shallow,  and 
not  infrequently  entirely  wanting. 
The  surface  is  nearly  one-third  less 
in  extent  than  the  buccal,  the  con- 
vergence of  the  mesial  and  distal 
surfaces  in  passing  to  the  lingual 
accounting  for  this  difference.  The 
occlusal  margin  is  formed  by  the 
double  incline  of  the  two  lingual 
cusps;  the  cervical  margin  is  either 

straight   or   slightly  concave   in    the   direction    of   the    occlusal 
surface,  while  the  mesial  and  distal  margins  are  rounded  and 

poorly  defined. 
''""^^'^°=^^  The   Mesial    Surface   of 

the  Crown  (Fig.  130). — This 
surface  is  inclined  to  flatness, 
with  a  slight  bulging  near  the 
center,  which  marks  the  point 
of  contact  with  the  approxi- 
mate tooth.  It  is  usually 
smooth,  and  unbroken  by  de- 
velopmental or  other  grooves, 
although  the  mesial  groove 
occasionally  traverses  it  after 
crossing  the  marginal  ridge 
from  the  occlusal  surface. 
Near  the  center  of  the  cervical  third  a  slight  concavity  is  often 
present.  The  margins  of  the  surface  are  somewhat  irregular, 
the  occlusal  margin  being  made  irregularly  concave  by  the  ridges 


Fig.   130.- 


-Lower  First   Molar,  Mesial 
Surface. 


230 


which  descend  from  the  two  mesial  cusps  ;  the  cervical  margin 
is  slightly  concave  in  the  direction  of  the  occlusal  surface,  and, 
while  the  buccal  margin  inclines  to  the  lingual  as  the  occlusal 
surface  is  approached,  the  lingual  is  almost  perpendicular. 

The  Distal  Surface  of  the  Crown  (Fig.  131). — Unlike  the 
mesial,  this  surface  is  possessed  of  a  decided  convexity  in  every 
direction.  It  is  surmounted  by  a  portion  of  the  distobuccal  and 
distolingual  cusps,  and  is  frequently  broken  by  the  distal  groove, 
which  reaches  it  after  crossing  the  marginal  ridge  from  the 
occlusal  surface.  The  occlusal  margin  is  irregularly  formed  ot 
the  marginal  ridges  which  descend  from  the  distobuccal  and 
distolingual  cusps;  the  cervical  margin  is  usually  straight,  while 

the  buccal  and  lingual  are 
rounded  and  indefinite. 

The  Neck  of  the 
Tooth.  —  One  character- 
istic feature  of  this  tooth  is 
the  greater  circumference 
of  the  crown  at  the  occlu- 
sal margin  over  that  at 
the  cervical  line,  giving  a 
flaring  appearance  to  the 
crown,  and  resulting  in  the 
production  of  a  neck  which 
is  much  constricted.  This 
is  particularly  noticeable 
when  looking  upon  the 
buccal  surface  of  the  tooth  ;  but  when  looking  upon  the  mesial 
or  the  distal  surface,  this  feature  is  not  so  pronounced,  although 
the  rather  heavy  fold  of  enamel  which  surmounts  the  cervical 
line  contributes  much  to  the  formation  of  the  neck  from  these 
aspects. 

The  Roots  of  the  Tooth. — The  roots  of  this  tooth  are  two 
in  number, — one  of  which  Is  placed  beneath  the  mesial,  and  the 
other  beneath  the  distal  half  of  the  crown, — and  are  named  the 
mesial  root  and  the  distah^oot.  The  fact  that  the  point  of  bifurca- 
tion is  constantly  in  close  proximity  to  the  neck  or  crown  of  the 
tooth  is  a  sufficient  reason  for  the  statement  that  two  roots  exist, 


Fig.   131. — Lower  First  Moi.ar,  Distal 
Surface. 


THE   LOWER   MOLARS.  231 

rather  than  a  single  root  with  two  branches.  The  roots  are  both 
mucli  flattened  from  mesial  to  distal,  and  broad  at  the  base  from 
buccal  to  lingual. 

Tlie  mesial  root  is  usually  the  larger  and  longer  of  the  two. 
After  leaving  its  base  it  generally  inclines  to  the  mesial,  but 
beyond  the  center  of  its  length  it  is  provided  with  a  distal  turn, 
which  in  some  instances  amounts  to  a  decided  crook.  The 
center  of  the  mesial  side  is  occupied  by  a  longitudinal  depres- 
sion, as  is  also  the  distal  side,  making  this  part  of  the  root  thin, 
giving  the  appearance  of  an  effort  to  bifurcate,  which  condition 
is  occasionally  present.  The  buccal  and  lingual  sides  of  the 
root  are  rounded  and  smooth,  and  taper  gradually  to  the  apex, 
which  is  somewhat  broadened  from  buccal  to  lingual. 

The  distal  root  is  usually  straight,  with  a  more  gradual  taper 
throughout,  ending  in  an  apical  extremity  more  pointed  than 
that  of  the  mesial  root.  A  longitudinal  depression  is  also 
present  upon  both  the  mesial  and  distal  sides,  but  is  never  so 
pronounced  as  that  upon  the  mesial  root.  The  buccal  and  lin- 
gual sides  are  convex  and  smooth.  The  root  possesses  little 
or  no  inclination  to  bifurcate. 


LOWER  SECOND    MOLAR. 

Calcification  Begins  about  the  Fifth  Year. 

Calcification  Completed,  Sixteenth  to  Seventeenth  Year. 
Erupts,  Twelth  to  Sixteenth  Year. 
Average  Length  of  Crown,  .27. 

Average  Length  of  Root,  .50. 

Average  Length  over  all,  .78. 

This  molar  differs  in  so  many  particulars  from  the  lower 
first  molar  that  a  separate  description  will  be  called  for.  The 
principal  variation  is  frequently  found  In  the  absence  of  the 
fifth  lobe  or  cusp,*  resulting  in  the  production  of  an  occlusal 
surface  much  less  complicated. 

The  Occlusal  Surface  of  the  Crown  (Fig.  132). — When 
the  crown  Is  studied  by  looking  directly  upon  this  surface,  the 

*When  five  cusps  are  present,  the  anatomy  of  this  surface  does  not  differ  from  that  of  the 
first  molar. 


ANATOMY. 


variations  between  this  and  the  first  molar  are  readily  noted. 
Four  equally  proportioned  cusps  are  observed,  separated 
from  each  other  by  four  developmental  grooves.  A  single  pit 
or  fossa  is  present,  the  four  grooves  arising  from  this  one  point. 
In  general  outline  two  principal  varieties  exist :  one  in  which  the 
opposite  sides  of  the  crown  are  nearly  of  the  same  length,  and 
parallel  with  each  other,  with  the  angles  rounded ;  the  other,  in 
which  either  the  buccal  or  lingual  margin  is  the  longest,  with  the 
mesial  and  distal  margins  converging  one  way  or  the  other,  as 
the  case  may  be.  The  marginal  ridges  are  formed  in  a  manner 
similar  to  those  of  the  first  molar,  with  the  exception  of  the  distal 
portion  of  the  buccal  ridge,  which  is  not  broken  by  a  develop- 
mental groove.  Each  mar- 
|  =  a  «o  ginal  ridge  is  divided  near 

°=5  °<^  its    center  by  one  of  the 

grooves  of  development, 
the  mesial  groove  crossing 
the  mesiomarginal  ridge, 
the  buccal  groove  crossing 
the  buccomarginal  ridge, 
the  lingual  groove  cross- 
ing the  lingual  ridge,  and 
the  distal  groove  passing 
over  the  distal  ridge.  In 
many  instances  numerous 
supplemental  grooves  are 
present,  which  in  turn  form  a  number  of  smaller  ridges.  The  four 
cusps  are  the  mesiobicccal ,  distobuccal,  mesiolingual,  and  distolin- 
gual.  In  a  general  way  they  are  similar  to  the  cusps  of  the  first 
lower  molar,  excepdng  that  they  are  somewhat  larger  and  prob- 
ably less  pointed  and  less  angular.  Each  cusp  is  provided  with 
a  number  of  ridges,  which  descend  from  the  summit  to  the  base, 
two  of  these  contributing  to  the  formation  of  the  marginal 
ridges,  one  passing  to  the  buccal  or  lingual,  and  one,  the  trian- 
gular ridge,  descending  the  central  incline  of  each  cusp.  The 
names  given  to  these  various  ridges  are  identical  with  those  of 
the  first  molar. 


32. — Lower  Second  Molar,  Occlusal 
Surface. 


THE   LOWER   MOLARS. 


al    Buccal    Mesiobuccal 
Groove        Cusp 


Fig.  133. — Lower  Second  Molar, 
Buccal  Surface. 


-the  buccal  pit. 


The    Buccal    Surface    of  the    Crown    (Fig.    133). — The 

principal  difterence  between  this  and  the  corresponding  surface 

of   the  first   molar  is   that  pro- 
duced by  the  absence  of  the  fifth 

cusp,  the  surface  being  divided 

into  two  parts  instead  of  three. 

The  single  division  is  caused  by 

the  buccal  groove,  which  reaches 

the    surface    after    crossing    the 

buccomarginal    ridge    from    the 

occlusal  surface.     The    position 

of  this  groove  is  usually  a  little 

to   the   mesial   of  the  center  of 

the    surface.       Like   the    buccal 

groove    of  the    first    molar,    it 

may    disappear   gradually  as   it 

passes  toward  the  cervical  line, 

or  it  may  end  in  a  well-marked  pit- 

The  Lingual    Surface  of  the    Crown   (Fig.    134). — This 
surface  so  closely  resembles  the  cor- 
1      I    I  responding  surface  of  the  first  molar 

iu  1  ■'^u  ^^^^  '^  ^^  somewhat  difficult  to  distin- 

c     u  s  guish  one  from  the  other.     The  oc- 

clusal margin  may  be  a  trifle  less 
irregular,  and  in  some  instances  more 
extensive,  than  the  buccal  surface, 
this  latter  feature  seldom  occurring 
in  the  first  molar. 

The  Mesial  Surface  of  the 
Crown  (Fig.  135). — This  surface 
corresponds  to  the  mesial  surface  of 
the  first  molar,  being  flattened  or 
slightly  convex  from  buccal  to  lingual, 
with  an  inclination  to  a  slight  depres- 
sion or  concavity  near  the  cervical 
margin. 
The  Distal  Surface  of  the  Crown  (Fig.  136). — On  account 

of  the  absence  of  the  fifth  cusp,  this  surface  is  less  complex  than 


Fig.  134.  —  Lower 
Second  Molar,  Lingual 
Surface. 


234 


that  of  the  first  molar.     It  is  convex  in  all  directions  ;  in  most 
instances  smooth,  in  others  broken  by  the  distal  groove,  which 


Fig.  135. — Lower  Second  Molar,  Mesial  Surface. 

reaches    it    after   crossing    the    distomarginal    ridge    from    the 
occlusal  surface. 

The  Roots  of  the  Tooth. — Like  the  first  molar,  these  are 


Mesial  Groove 


Fig.  136. — Lower  Second  Molar,  Distal  Surface. 


two  in  number,  a  mesial  and  a  distal.      They  are  much  less  con- 
stant in  form,  are  often   nearer  together,  and  in  some  instances 


THE   LOWER    MOLARS. 


235 


united.  When  the  two  roots  exist, — which  may  be  considered 
the  normal  condition, — they  are  less  flattened  upon  their  mesial 
and  distal  sides,  with  the  longitudinal  depression  wanting  or  but 
slightly  apparent.  These  roots,  therefore,  are  more  rounded  in 
general,  taper  more  gradually  from  neck  to  apex,  and  end  in  a 
rounded  apex,  this  often  being  provided  with  a  slight  distal 
curve. 


LOWER  THIRD  MOLAR. 

Calcification  Begins,  Eighth  to  Ninth  Year. 

Calcification  Completed,  Eigthteenth  Year. 
Erupts,  Sixteenth  to  Twentieth  Year. 
Average  Length  of  Crown,  .26. 

Average  Length  of  Roots,  .36. 

Average  Length  Over  All,  .62. 

This  tooth  is  probably  subject  to  a  greater  variety  in  form 
than  any  other  tooth  in  the  mouth.  There  are,  however,  two 
varieties  which  are  most  frequently  met  with.  In  one  the  crown 
of  the  tooth  is  similar  to  the  lower  second  molar,  being  pro- 
vided with  four  cusps,  which 

are  separated  from  oneanother  "s;;  glji, 

byfour  developmental  grooves  .b^  ~|5 

(Fig.  137).  The  other  is  simi- 
lar to  the  lower  first  molar, 
having  five  cusps  and  five  de- 
velopmental grooves. 

While  these  two  forms  are 
those  most  commonly  met  with, 
the  occlusal  surface  may  be 
so  broken  by  numerous  sup- 
plemental and  developmental 
grooves  that  even  six  or  eight 
well-defined  cusps  may  be  pre- 
sent.  Whatever  complications 

may  exist  upon  the  occlusal  surface,  a  central  fossa  is  usually  pre- 
sent, from  which  radiate  the  various  developmental  grooves. 
When  the  central  fossa  is  absent,  the  space  which  it  should  occupy 
is  usually  taken  up  bya  rounded  cusp,  by  the  interference  of  which 
the  grooves  are  prevented  from  uniting,  and  their  course  is  much 


137. — Lower  Thiru  Molar, 
Occlusal  Surface. 


236  ANATOMY. 

distorted.  Along  with  these  variations,  the  tooth  is  subject  to 
much  variety  in  size.  In  some  instances  the  crown  is  one-third 
less  in  circumference  than  that  of  either  the  first  or  second 
lower  molars,  while  in  others  it  is  a  trifle  greater.  The  in- 
crease in  the  size  of  the  crown  is  generally  accompanied  by  an 
increase  in  the  number  of  cusps.  One  feature  very  common 
to  the  crown  is  its  inclination  to  the  circular  form,  almost  result- 
ing in  the  absence  of  the  angles  common  to  molars  in  general. 
The  marginal  ridges  are,  of  course,  subject  to  the  ever-varying 
conditions  to  be  found  upon  the  occlusal  surface  ;  in  general, 


Disto- 

Disto-    Mesio- 

lingual 

buccal    buccal 

Cusp 

Cusp      Cusp 

Fig.  138. — Lower  Third  Molar, 
Buccal  Surface. 


Fig.  139. — Lower  Third  Molar, 
Lingual  Surface. 


they  are  poorly  defined,  and  are  frequently  crossed  by  numerous 
small  supplemental  grooves,  dividing  them  into  many  minute 
tubercles.  The  latter  are  smooth  and  strongly  convex,  with 
their  general  outlines  much  influenced  by  the  number  of  cusps. 
The  Roots  of  the  Tooth. — While  this  tooth  is  strongly 
inclined  to  be  two-rooted,  like  the  other  lower  molars,  this 
condition  is  by  no  means  the  common  one.  Like  the  crown, 
the  roots  are  probably  more  variable  than  those  of  any  other 
tooth.  A  single  conic  root  may  be  present,  or  a  mesial  and 
a  distal   root  may  exist ;  again,  the  mesial   root  may  bifurcate, 


THE   LOWER   MOLARS. 


237 


thus  resulting  in  three.  In  some  instances,  four,  or  even  five, 
branches  may  be  given  off  from  a  common  base.  When 
more  than  two  roots  are  present,  they  are  usually  much  twisted 


RT  If  f'J 


Fig.   140. — Types  of  Lower  Third  Molars 


or  crooked,  and,  while  generally  inclined  to  the  distal,  are  liable 
to  branch  in  various  directions. 

A  better  idea  in  the  variations  in  this  tooth  may  be  had  from 
the  accompanying  illustration  (Fig.  140). 


CHAPTER  X. 
THE   PULP-CAVITIES  OF  THE  TEETH. 

In  the  preceding  chapters  the  study  of  the  teeth  has  been 
confined  to  their  external  forms  ;  it  will  now  be  necessary  to 
learn  something  of  their  internal  anatomy,  and  for  this  purpose 
various  dissections  of  each  individual  tooth  must  be  made. 

Dissections. — First,  a  longitudinal  dissection  of  each  tooth 
should  be  made  by  sawing  or  filing  from  labial  to  lingual  in  the 
anterior  teeth,  or  from  buccal  to  lingual  in  the  posterior  teeth. 
Second,  a  longitudinal  dissection  by  sawing  from  mesial  to 
distal.  Third,  numerous  transverse  dissections  by  sawing  through 
the  crown  or  root  at  various  points. 

These  dissections  will  expose  to  view  a  central  cavity  with 
outlines  closely  corresponding  to  those  of  the  tooth  itself.  This 
is  called  ^& pulp-cavity,  and  in  the  vital  tooth  contains  the  forma- 
tive and  life-sustaining  substance  of  the  dentin,  the  dental-pulp. 
The  pulp-cavity  is  divided  into  two  principal  parts,  that  portion 
within  the  crown  of  the  tooth  being  l\\& p7ilp-ckamber,  while  that 
traversing  the  root  is  the  pulp-canal.  At  the  apex  of  the  root 
the  canal  ends  in  a  small  foramen,  the  apical  foramen,  which 
transmits  the  blood-vessels  and  nerves  to  the  pulp.  The  pulp- 
chamber  occupies  the  center  of  the  crown  and  is  always  a  single 
cavity  ;  the  pulp-canals  are  prolongations  from  this  central  cavity, 
and  are  usually  one  for  each  root,  although  in  some  instances  two 
or  more  canals  are  present  in  a  single  root.  The  form  of  the 
pulp-chamber  varies  with  the  shape  of  the  crown,  the  outline  of 
the  cutting-edge  in  the  incisor  teeth  being  reproduced  in  that 
part  of  the  chamber  nearest  to  the  cutting-edge,  wdiile  in  the 
bicuspids  and  molars  the  occlusal  surface  is  reproduced  on  the 
wall  of  the  pulp-chamber,  immediately  beneath  it,  the  lateral 
walls  corresponding  to  the  various  sides  of  the  tooth.  In  the 
incisors  and  cuspids  the  pulp-chamber  passes  so  gradually  into 
the  pulp-canal  that  a  positive  line  of  demarcation  between  the  two 


THE   PULP-CAVITIES   OF   THE   TEETH.  239 

is  not  observed.  In  the  bicuspids  and  molars  the  canals  may 
be  readily  distinguished  by  a  sudden  constriction  and  branching 
out  of  the  cavity  into  the  various  roots,  which  prolongations 
gradually  decrease  in  size  until  the  apical  foramen  is  reached. 
The  size  of  the  pulp-cavity  is  much  influenced  by  the  age  of  the 
tooth,  its  functional  activity,  character  of  the  occlusion,  etc. 
The  tooth-pulp,  as  the  formative  organ  of  the  dentin,  gradually 
decreases  in  size  as  the  tooth  develops  (see  Development  of  the 
Teeth),  and  as  a  result  of  this  action  the  youngest  teeth  are  pro- 
vided with  the  largest  pulp-cavities.  At  the  time  of  eruption  of 
a  tooth,  the  diameter  of  the  pulp-cavity  is  about  equal  to  one- 
half  the  diameter  of  the  crown,  while  the  length  of  the  canal 
must,  of  necessity,  accord  with  the  extent  of  root-calcification. 
As  the  growth  of  the  tooth  proceeds,  the  diameter  of  both  the 
chamber  and  canal  is  gradually  diminished  ;  this  gradual  reduction 
in  size  is  continued  during  the  life  of  the  tooth,  and  if  permitted  to 
proceed  until  old  age,  the  chamber  and  canal  may  become  almost 
or  entirely  obliterated.  It  must  be  remembered  that  while  the 
diameter  of  the  root-canal  is  diminished  with  the  growth  of  the 
tooth,  its  length  increases,  continuing  to  do  so  until  the  time  of 
complete  root-calcification.  During  the  period  of  root-develop- 
ment the  diameter  of  the  root-canal  is  greatest  at  the  free  or  apical 
end  of  the  root,  at  which  point  it  presents  a  funnel-shaped  opening 
(Fig.  141).  As  the  root  continues  to  calcify,  this  funnel-shaped 
extremity  of  the  canal  advances  in  the  direction  of  calcifica- 
tion, and  finally,  as  the  formative  process  nears  completion,  the 
mouth  of  the  funnel  gradually  disappears,  and  the  apical  fora- 
men is  established.  The  various  lobes  of  the  teeth  are  pene- 
trated by  a  prolongation  of  the  pulp-cavity,  these  being  called 
the  horns  of  the  pulp-chamber.  The  depth  to  which  the  horn 
penetrates  the  lobe  varies  in  accordance  with  the  form  of  the 
latter.  If  the  tooth  is  one  provided  with  long,  penetrating 
cusps,  the  horns  of  the  pulp-chamber  will  also  be  long,  but  if 
the  cusps  be  poorly  formed,  the  horns  of  the  chamber  will  be 
short.  In  the  anterior  teeth,  when  the  lobal  construction  is  put- 
lined  by  well-marked  developmental  grooves,  the  horns  of  the 
pulp-chamber  will  be  three  in  number  and  directed  toward  the 
cutting-edge.      These    are   most   marked   in    young    teeth,   and 


gradually  disappear  as  age  advances.  The  functional  activity 
of  the  teeth  also  serves  to  materially  reduce  the  size  of  the  pulp- 
chamber.  Thus,  when  opposing  teeth  occlude  squarely  and 
firmly  against  each  other,  with  more  or  less  rubbing  or  sliding 
during  mastication,  the  external  surface  is  prone  to  rapid 
abrasion,  and,  as  a  direct  result  of  this  external  change,  the 
pulp-chamber  undergoes  a  corresponding  alteration  by  a  growth 
of  secondary  dentin  about  its  walls. 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH. 

Upper  Central  Incisor. — Figure  141  represents  a  num- 
ber of  labiolingual  sections  presenting  the  relative  size  and 
shape  of  the  pulp-cavity  in  the  upper  central  incisor  at  various 


Fig.   141. — The   Pulp-cavity  in   the  Upper   Central   Incisor,  from  the  Sixth  to 
THE  Tenth  Year. 

ages.  In  No.  i  the  condition  existing  at  about  the  sixth  year,  or 
at  a  time  immediately  prior  to  the  eruption  of  the  tooth,  is  shown. 
The  tooth-crown  is  fully  formed  and  calcified  ;  the  cervical  line 
may  be  observed,  as  well  as  a  small  portion  of  the  root-wall. 
The  pulp-chamber,  which  is  represented  by  the  dark  portion  ot 
the  cut,  occupies  about  one-third  of  the  diameter  of  the  crown 
at  its  greatest  width.  The  pulp-chamber  at  this  age,  when 
viewed  in  this  direction,  forms  almost  a  perfect  cone,  the  base 
of  which  is  directed  upward  or  toward  the  future  extremity  ot 
the  root,  and  its  apex  downward  in  the  direction  of  the  cutting- 
edge  of  the  tooth.     The  apex  of  the  cone  may  end  somewhat 


THE   PULP-CAVITIES   OF   THE   UPPER   TEETH.  241 

abruptly,  or  it  may  be  lengthened  into  a  slender,  horn-like  pro- 
jection, extending-  well  toward  the  cutting-edge.  No.  2  repre- 
sents the  same  tooth  about  the  seventh  year,  or  at  a  time 
shortly  after  its  eruption.  The  pulp-chamber  has  become 
slighdy  reduced  in  its  basal  diameter,  while  but  little  change 
has  taken  place  in  the  apex.  That  portion  of  the  pulp-cavity 
above  the  cervical  line  represents  a  part  of  the  future  pulp- 
canal.  At  this  age  the  canal  is  a  direct  continuation  of  the 
conic  pulp-chamber,  ending  above  in  a  broad,  funnel-shaped 
extremity.  No.  3  shows  the  condition  of  the  cavity  about 
the  eighth  year.  The  diameter  of  the  pulp-chamber  is  con- 
siderably diminished,  the  apex  has  slightly  receded,  and  the 
hornlike  projection  has  partly  disappeared.  The  increase  in 
the  length  of  the  canal  is  about  fV  of  an  inch  over  its  length 
at  seven  years.  The  two  parallel  sides  of  the  canal  have 
lengthened  proportionately,  and  the  funnel-shaped  extremity 
is  reduced  in  diameter  owing  to  the  gradual  narrowing  of  the 
root-walls.  No.  4  gives  the  relative  size  of  the  pulp-chamber 
and  canal  at  the  ninth  year,  or  at  a  time  when  root- calcifica- 
tion is  nearing  completion.  The  decrease  in  the  capacity 
of  the  chamber  is  readily  apparent ;  the  horn-like  projection 
has  disappeared  and  the  parallel  sides  of  the  canal  are  partly 
extended  into  the  chamber,  thus  reducing  the  length  of  the  cone. 
In  the  canal  a  greater  reduction  has  taken  place  in  its  diameter, 
while  its  length  has  increased  about  j^  of  an  inch  over  that 
at  eight  years,  and  the  diameter  of  the  funnel-shaped  open- 
ing is  but  little  greater  than  that  of  the  body  of  the  canal. 
No.  5,  which  represents  the  tooth  about  the  tenth  year,  shows 
the  calcific  action  in  the  root  completed,  and  the  apical  foramen 
established.  A  glance  at  the  illustration  will  show  the  gradual 
decrease  in  the  capacity  of  the  pulp-cavity  and  the  completion 
of  its  growth  in  an  apical  direction.  At  this  stage  of  develop- 
ment the  fan-shaped  extremity  of  the  canal  gradually  disappears, 
and  for  the  first  time  in  the  life  of  the  tooth  the  canal  partakes 
of  the  external  root  form  throughout  its  entire  extent. 

Figure  142,  A,  represents  the  size  and  form  of  the  average 
pulp-cavity  in  the  adult  upper  central  incisor.  In  its  entirety 
it  represents  a  double   cone,   with   a   common    base    near    the 


cervical  line,  the  pulp-cavity  forming  one  cone  and  the  pulp- 
canal  the  other.  At  this  common  base  the. cavity  assumes  its 
largest  diameter,  which  measurement  is  approximately  equal  to 
one-fourth  the  labiolingual  diameter  of  the  tooth.  The  extent 
and  form  of  the  lower  cone,  or  that  represented  by  the  pulp- 
chamber,  varies  in  the  adult  tooth  with  the  tooth  type.  Thus,  in 
the  nervous  type  the  cone  is  long  and  narrow,  with  the  apex 
ending  in  a  hair-like  projection.  In  the  tooth  of  the  lymphatic 
temperament  the  cone  is  prone  to  be  wide,  with  its  apex  ending 
abruptly.  In  the  sanguine  and  bilious  types  the  form  and 
extent  of  the  cone  do  not  partake  of  either  of  the  foregoing  ex- 
tremes, but,  in  keeping  with  the  outline  of  the  crowns,  are  inter- 
mediate between  them.     Figure  142,  B,  represents  the  average 


142. — Pulp-cavity  in  the  Upper  Central  Incisor. 


condition  of  the  pulp-cavity  in  the  central  incisor  in  advanced 
age,  and  shows  a  general  reduction  in  the  size  of  both  chamber 
and  canal.  A  further  study  of  the  pulp-chamber  and  canal  may 
be  made  by  a  mesiodistal  section  made  through  the  long  axis 
of  the  tooth  (Fig.  142,  C).  The  outline  of  the  cavity,  viewed  in 
this  way,  closely  follows  the  outline  of  the  crown  and  root  of  the 
tooth.  There  is  no  distinct  division  between  the  chamber  and 
the  canal,  the  former  gradually  blending  into  the  latter.  The 
outline  of  the  entire  cavity  is  that  of  a  single  cone,  with  its  base 
directed  toward  the  cutting-edge  and  its  apex  in  the  direction 
of  the  apical  extremity  of  the  root.  The  lower  margin  of  the 
pulp-chamber,  or  that  nearest  the  cutting-edge  of  the  crown,  is 


THE    rULP-CAVITIES    OF    THE    UPPER    TEETH.  243 

broad  from  mesial  to  distal  and  thin  from  labial  to  lingual.  This 
margin  in  the  average  adult  tooth  is  about  on  a  line  with  the 
center  of  the  labial  surface  of  the  crown,  and  the  lateral  walls  of 
the  cavity  as  they  pass  upward  converge  slightly,  and  finally 
blend  into  the  walls  of  the  canal  at  a  point  somewhat  beyond 
the  cervical  line.  During  the  early  life  of  the  tooth  the  margin 
of  the  chamber  nearest  the  cutting-edge  presents  three  well- 
defined  horns,  corresponding  to  the  three  rudimentary  lobes 
found  upon  the  cutting-edge  at  this  period.  These  horns 
rapidly  disappear,  and  are  seldom  found  after  the  fifteenth 
year.  In  certain  tooth  types,  however,  the  mesial  and  distal 
horns  may  continue  present  until  adult  age,  and  even  into 
middle   life,    but   when    this   occurs   it  is    not  the  result  of   the 


Fig.  143. — Transverse  Sections,  Root  of  Upper  Central  Inxisors,  Slightly 
Enlarged. 

temporary  tooth  form,  but  is  occasioned  by  the  permanent 
angular  outline  of  the  crown. 

Figure  143  represents  a  number  of  transverse  sections  of  an 
upper  central  incisor,  showing  the  outline  and  relative  size  of 
the  pulp-cavity  in  passing  from  the  base  of  the  crown  toward 
the  apex  of  the  root.  No.  i  shows  the  oudine  of  the  cavity  at 
the  cervical  line  ;  No.  2  represents  the  condition  y^  of  an  inch 
nearer  the  apex  of  the  root  ;  No.  3  is  from  the  center  of  the 
root  length,  while  No.  4  is  from  the  region  of  the  apex. 

Upper  Lateral  Incisor. — The  pulp-cavity  in  the  upper 
lateral  incisor  is  so  nearly  identical  with  that  of  the  central  that  it 
will  only  be  necessary  to  call  attention  to  one  or  two  points 
which  are  at  variance.  Figure  1 44  shows  the  five  stages  as  repre- 
sented by  the  growth  of  the  tooth.  In  general  it  will  be 
observed  that  the  cavity  is  much  smaller  than  that  of  the 
central  incisor,  but  this  difterence  is  to  be  accounted  for  in  the 


smaller  proportions  of  the  tooth.     No.  i   shows  the  condition 
of  the  crown  and  pulp-cavity  about  the  sixth  year,   the  pulp- 


FlG.    144.  —  PULP-CAVITY     IN    THE     UPPER     LATERAL     InCISOR,    FROM    THE   SiXTH    TO   THE 

Tenth  Year. 


cavity  occupying  a  large  portion  of  the  partly  calcified  tooth- 
crown.     No.  2  represents  the  conditions  present  at  the  seventh 


Fig.   145. 


year,  or  about  the  time  of  the  eruption  of  the  tooth.     The  pulp- 
chamber  at  this  age  resembles  a  perfect  cone,  the  base  of  which 


THli   PULP-CAVITIES   OF   THE    UPPER   TEETH.  245 

reaches  to  the  root-walls,  and  faintly  outlines  the  beginning  of 
the  future  pulp-canal.  In  No.  3,  at  eight  years,  the  length 
of  the  root  has  increased  about  ^  of  an  inch,  and  the  par- 
allel sides  of  the  walls  of  the  pulp-canal  have  made  their 
appearance.  In  No.  4,  at  nine  years,  by  the  growth  of  the  root 
the  canal  has  considerably  increased  in  length  and  at  the  same 
time  much  decreased  in  diameter,  while  in  No.  5,  at  ten  years, 
the  root  is  completely  formed,  the  apical  foramen  established, 
and  the  maximum  size  of  the  entire  pulp-cavity  in  the  fully 
formed  tooth  shown. 

Figure  145,  A,  shows  the  average  condition  of  the  pulp-cavity 
in  the  upper  lateral  incisor  at  adult  age,  while  figure  145,  B, 
represents  the  same  tooth  in  old  age.  In  a  mesiodistal  section — 
figure  145,  C — a  very  close  resemblance  to  the  pulp-cavity  in  the 
central  incisor  will  be  noticed.  While  the  pulp-cavity  is  smaller 
than  that  of  the  central  incisor,  it  is  usually  a  trifle  larger  in 
proportion  to  the  size  of  the  tooth.  Owing  to  the  marked 
constriction  at  the  neck  of  this  tooth,  there  is  occasionally  found 
a  slight  line  of  distinction  between  the  pulp-chamber  and  canal, 
but  in  the  majority  of  instances  this  is  not  to  be  observed.  The 
horns  of  the  pulp-chamber  are  in  every  respect  similar  to  those 
of  the  central  incisor,  excepting  w^hen  they  exist  permanently, 
in  which  case  the  mesial  horn  is  usually  the  longest.  By  the 
transverse  sections  shown  in  figure  145,  the  gradual  decrease  in 
size  and  change  in  form  in  the  root-canal  are  presented,  the 
sections  being  similar  to  those  made  in  the  root  of  the  central 
incisor. 

Upper  Cuspid. — The  pulp-cavity  of  this  tooth  is  in  gen- 
eral similar  to  that  of  the  incisors,  excepting  that  the  coronal 
extremity  of  the  chamber  is  conic  and  inclined  to  a  horn- 
like projection  which  penetrates  the  single  cusp  of  the  tooth- 
crown  in  the  direction  of  its  summit.  Figure  146  represents 
a  number  of  labiopalatal  sections.  No.  i  shows  the  condi- 
tion of  the  pulp-cavity  about  the  seventh  year,  or  fully  five 
years  before  the  eruption  of  the  tooth.  The  pulp-chamber 
partakes  of  the  cone  shape  previously  referred  to,  but  the  mar- 
gins, instead  of  being  straight  lines,  are  somewhat  bowed  or 
concave,   thus    conforming  more  closely  to  the  outline  of  the 


246  ANATOMY. 

crown.  The  central  horn  of  the  chamber  is  proportionately 
longer  than  that  of  the  incisors,  in  correspondence  with  the  cusp 
of  the  tooth.  At  this  age  the  formative  process  has  barely 
extended  to  the  root-walls  ;  therefore,  the  width  of  the  cavity  is 
about  equal  to  its  length.  In  No.  2,  at  eight  years,  an  increase 
in  the  capacity  of  the  chamber  over  that  of  the  incisors  is 
shown,  this  being  the  result  of  the  greater  bulk  in  the  tooth- 
crown.  The  cone-like  outline  of  the  chamber  is  somewhat 
broken  by  an  effort  of  its  margins  to  follow  the  outline  of  the 
crown.  In  No.  3,  at  nine  years,  the  principal  change  has  taken 
place  in  the  canal,  which  has  lengthened  fully  y^^-  of  an  inch, 
and  the  funnel-shaped    extremity,   instead  of  joining  with  the 


Fig.  146. — Pulp-cavity  in  the  Upper  Cuspid,  from  the  Seventh  to  the  Twelfth 

Year. 


pulp-chamber  direct,  is  continued  below  by  two  parallel  walls 
to  the  true  beginning  of  this  cavity.  At  ten  years.  No.  4,  a 
more  marked  transformation  has  taken  place  in  both  portions 
of  the  cavity.  The  diameter  of  the  chamber  at  the  cervical  line 
has  diminished,  as  has  also  the  length  of  the  cone.  The 
increase  in  the  length  of  the  root,  which  has  been  proportion- 
ately greater  than  that  of  the  preceding  year,  has  extended 
the  length  of  the  canal  about  j{  of  an  inch.  The  walls 
of  the  canal  are  no  longer  parallel  with  each  other,  but  are 
inclined  to  follow  the  root-outlines.  The  funnel-shaped  opening 
is  much  reduced  both  in  length  and  breadth.  No.  5  represents 
the  condition  at  the  time  of  the  eruption  of  the  tooth,  or  about 
the  twelfth  year.     The    general   outline    of   the    pulp-cavity  is 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  247 

that  of  a  double  cone,  with  a  common  base  at  a  point  nearly 
corresponding  to  the  cervical  line.  The  diameter  in  both  the 
chamber  and  canal  has  considerably  decreased,  while  the  central 
horn  in  the  former  has  further  receded.  The  calcification  of 
the  root  externally  is  about  complete  and  the  foramen  formed. 
In  this  particular  the  cuspid  tooth  differs  from  the  incisors,  and 
in  fact  from  all  other  teeth,  in  having  its  root-calcification  about 
completed  and  the  apical  foramen  established  at  or  soon  after  the 


Fig.    147. 


time  of  its  eruption.  Figure  1 47,  A,  gives  an  idea  of  the  capacity 
of  the  pulp-cavity  in  the  upper  cuspid  at  maturity,  while  figure 
146,  B,  shows  the  condition  in  advanced  age.  Figure  147,  C,  is  a 
mediodistal  section  of  a  matured  upper  cuspid.  The  coronal 
extremity  of  the  pulp-chamber  is  square,  and  but  little  inclined 
to  follow  the  outline  of  the  mesial  and  distal  cutting-edges. 
The  chamber  passes  into  the  canal  without  a  mark  of  separa- 
tion, and  the  latter  gradually  diminishes  in  diameter  as  the  apex 
of  the  root  is  approached.  At  its  point  of  beginning  the  canal 
is  sometimes  inclined  to  flatness  from   mesial  to  distal,  but  in 


248  ANATOMY. 

passing  toward  the  apex  this  tendency  disappears,  and  it  becomes 
more  circular  in  outline.  In  figure  147,  D,  a  transverse  sec- 
tion through  the  tooth  at  the  cervical  line  is  shown,  giving 
an  idea  of  the  proportionate  size  and  form  of  the  canal  in  the 
adult  tooth,  while  E,  F,  and  G  represent  transverse  sections 
through  the  root  of  the  same  tooth  at  various  points  between 
the  cervical  line  and  the  apex  of  the  root. 


Fig.   148. — Pulp-cavities  of  the  Upper  First  Bicuspid,  from  the  Seventh  to  the 
Twelfth  Year. 

Upper  First  Bicuspid. — The  study  of  the  pulp-cavity  in 
this  tooth  differs  in  many  particulars  from  that  of  the  incisors 
and  cuspids.  First,  the  line  of  distinction  between  the  pulp- 
chamber  and  the  root-canal  or  canals  is,  in  most  instances,  defi- 
nitely marked  by  the  bifurcation  of  the  roots  and  a  corresponding 
branching  of  the  pulp-cavity  into  two  fine  canals,  one  of  which 
occupies  the  center  of  each  root.  This  division  of  the  cavity 
brings  the  center  of  the  pulp-chamber  almost  on  a  level  with  the 
cervical  line.  In  figure  148,  No.  i  shows  the  partly  calcified 
crown  of  the  upper  first  bicuspid  at  the  seventh  year.  A  por- 
tion of  the  pulp-chamber  alone  may  be  studied  at  this  period, 
and  this  is  found  to  be  somewhat  irregular  in  outline,  with  a 
broadened,  funnel-shaped  opening  above,  and  two  small,  cone- 
like projections  below,  pointing  into  either  cusp  of  the  crown. 
These  latter  projections  are  the  horns  of  the  pulp-chamber,  and 
are  named  in  accordance  with  the  cusp  which  they  occupy.  In 
very  young  teeth  it  is  not  unusual  to  find  these  horns  penetrat- 
ing the  dentin  almost  to  the  enamel-wall.  No.  2,  at  eight  years, 
shows  the  crown  fully  calcified  and  the  outline  of  the  base  of  the 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  249 

roots  established.  The  horns  of  the  pulp-chamber  have  slightly 
receded,  and  the  branching  of  the  canals  is  made  manifest  by  the 
central  deposit  of  dentin.  In  No.  3,  at  nine  years,  the  capacity 
of  the  pulp-chamber  is  much  decreased,  and  appears  to  have 
receded  bodily  rootward.  The  roots  are  calcified  to  about  one- 
third  their  full  length,  and  the  canals  which  traverse  them  are 
each  provided  with  the  funnel-shaped  opening  at  their  free  cal- 
cifying extremities.  In  No.  4,  at  ten  years,  the  decrease  in  the 
size  of  the  pulp-chamber  is  not  only  caused  by  the  deposit  of 
dentin  upon  the  occlusal  and  lateral  walls,  but  from  the  direction 
of  the  roots  as  well.  The  diameter  of  the  root  canals  is  much  less 
than  at  nine  years,  but  the  walls  are  as  yet  parallel.  No.  5 
shows  the  roots  fully  formed  and  the  apical  foramina  established, 
which  condition  occurs  about  the  twelfth  year.  The  horns  of 
the  pulp-chamber  have  receded  somewhat,  and  the  center  of  this 
cavity  is  now  almost  on  a  level  with  the  cervical  line.  The  canals 
have  assumed  the  form  of  the  roots  themselves,  and  their  diam- 
eter is  much  diminished.  The  illustration  shows  the  propor- 
tionate maximum  size  of  the  chamber  and  canals  in  this  tooth 
after  completion  of  surface  calcification.  It  will  be  observed 
that  the  foramina  are  proportionately  smaller  than  those  of  the 
incisors  and  cuspids  at  a  corresponding  period,  this  condition 
resulting  from  the  lesser  diameter  of  the  roots. 

Figure  149,  A,  illustrates  the  approximate  size  and  form  of 
the  pulp-chamber  and  canals  at  adult  age,  and  attention  is  called 
to  the  appearance  of  the  horns  of  the  pulp-chamber.  It  will  be 
observed  that  the  horn  which  penetrates  the  buccal  cusp  is 
larger  and  more  pointed  than  that  directed  toward  the  lingual 
cusp  ;  this  condition  is  fully  explained  by  the  buccal  cusp  being 
proportionately  larger  and  longer  than  the  lingual.  In  the  same 
figure,  B  represents  the  pulp-cavity  in  the  first  upper  bicuspid 
at  advanced  age.  The  foregoing  description  applies  only  to  the 
two-rooted  bicuspids,  but  as  many  of  these  teeth  have  but  one 
root,  an  additional  description  will  be  necessary.  When  a  single 
root  is  present,  many  varieties  in  the  outline  of  the  pulp-cavity 
will  be  presented  ;  this  variation,  however,  seldom  affects  the 
capacity  or  form  of  the  pulp-chamber.  Two  distinct  canals  may 
exist  in  the  single  root  (Fig.    149,   C),  branching  off  from   the 


ANATOMY. 


chamber,  one  from  the  buccal  and  one  from  the  hngual  portion. 
These  canals  gradually  taper  in  the  direction  of  the  apex  of  the 
root,  and  may  end  in  a  single  foramen,  or  in  distinct  foramina. 
Occasionally  the  canals  will  unite  before  reaching  the  root- 
apex,  and  continue  as  a  single  canal  ending  in  a  single 
foramen,  or  they  may  communicate  at  one  point  and  again 
diverge  and  finally  end  in  separate  foramina.  In  some 
instances  the  pulp-canal  appears  to  be  a  direct  continua- 
tion   of   the  pulp-chamber,    extending    throughout    the    length 


Fig.  149. — Pulp-cavities  of  the  Upper  First  Bicuspid,  Enlarged  about  One-third. 

of  the  root  in  the  form  of  a  flattened  canal,  with  its 
greatest  diameter  from  buccal  to  lingual  (Fig.  149,  D). 
When  two  separate  canals  exist  in  the  single  root,  the  outward 
appearance  of  the  root  indicates  a  near  approach  to  two  roots ; 
when  the  single  flattened  canal  is  present,  the  root  is  also 
flattened  and  shows  no  sign  of  bifurcation.  Reference  has  been 
made  to  the  horns  of  the  pulp-chamber,  and  in  this  connection 
it  will  be  well  to  speak  of  the  extent  to  which  they  may  exist. 
In  that  type  of  tooth  provided  with  long  penetrating  cusps  the 
horns  will  dip  well  down  into  the  cusp  occasionally  to  the  full 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  251 

depth  of  the  dentin,  and  in  rare  instances  may  penetrate  the 
enamel.  In  those  teeth  ladling  in  cusp-formation  the  length 
of  the  horns  will  be  correspondingly  reduced,  and  may  be 
entirely  wanting.  In  the  two-rooted  upper  first  bicuspid  the 
floor  of  the  pulp-chamber,  or  that  part  of  the  cavity  directed 
rootward,  is  prominent  and  rounded  in  the  center,  from 
which  point  it  gradually  slopes  toward  the  entrances  to  the 
canals,  one  of  which  arises  from  the  extreme  buccal  margin,  and 
the  other  from  the  extreme  lingual  margin.  Figure  149,  E, 
represents  a  transverse  section  of  the  two  roots  immediately 
below  the  point  of  bifurcation.  F  represents  a  section  of  the  two 
roots  midway  between  the  cervical  line  and  the  apical  extremity, 
while  G  is  a  transverse  section  of  D  at  the  cervical  line. 


Fig.    150. — PuLPrCAViTiEs   in   the   Upper   Second   Bicuspid,  from   the   Seventh   to 
THE  Twelfth  Year. 


Upper  Second  Bicuspid. — The  pulp-cavity  of  this  tooth 
is  in  many  respects  similar  to  that  of  the  first  bicuspid,  the  prin- 
cipal variations  being  in  the  horns  of  the  chamber,  which  are 
proportionately  smaller  in  correspondence  with  the  diminution 
in  cusp-formation.  There  is  usually  no  positive  line  of  demarca- 
tion between  the  chamber  and  canal,  the  latter  being  quite 
large,  and  broad  from  buccal  to  lingual.  The  extent  of  the  pulp- 
chamber  is  sometimes  well  defined  by  the  presence  of  two  root- 
canals,  similar  to  those  described  in  connection  with  the  first 
bicuspid.  In  rare  instances  the  tooth  may  possess  two  roots, 
each  of  which  would  be  traversed  by  a  canal.  Figure  1 50  repre- 
sents the  various  stages  of  the  development  of  the  pulp-cavit}', 
as  shown  by  a  longitudinal  section  from  buccal  to  lingual.  No.  i 
shows  the  condition  at  seven  years,  or  at  a  time  when  a  portion 


of  the  crown  only  is  calcified,  in  consequence  of  which  the  pulp- 
chamber  alone  can  be  studied  at  this  period.  The  buccal  and 
lingual  horns  of  the  chamber  may  be  observed  penetrating  the 
dentin  in  the  direction  of  their  respective  cusps.  No.  2  shows  the 
advance  made  in  the  formative  process  by  the  eighth  year,  or  at  a 
time  immediately  prior  to  the  eruption  of  the  tooth  ;  the  outline 
of  the  chamber  is  completed  and  the  walls  of  the  future  pulp- 
canal  faintly  outlined.  At  this  period  there  has  been  but  little 
change  in  the  horns  of  the  pulp-cavity.     No.  3  shows  the  condi- 


FiG.  151. — Section  of  Upper  Second  Bicuspid,  Slightly  Enlarged. 

tion  of  the  tooth  at  the  ninth  year,  or  at  the  beginning  of  its 
eruptive  period.  The  diameter  of  the  chamber  has  somewhat 
decreased,  the  horns  have  slightly  receded,  the  funnel-shaped 
extremity  of  the  cavity  has  advanced  beyond  the  cervical  line, 
and  is  now  confined  to  the  canal  alone.  No.  4  represents  the 
condition  of  the  pulp-cavity  about  the  tenth  year.  A  gradual 
decrease  in  the  diameter  of  both  the  chamber  and  canal  is 
observed,  and  the  horns  of  the  pulp-cavity  are  growing  less 
prominent.  The  length  of  the  root  having  increased  nearly 
one-quarter  of  an  inch,  we  find  a  corresponding  addition  to  the 
length  of  the  canal.      No.  5    shows  the   maximum   size  of  the 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  253 

pulp-cavity  in  the  upper  second  bicuspid,  which  condition 
accompanies  the  completion  of  the  external  calcific  action  at 
the  twelfth  year.  As  previously  stated,  the  cavity,  in  its 
entirety,  presents  no  line  of  separation  between  the  chamber 
and  canal,  but  gradually  tapers  from  its  broadened  base  in  the 
crown  to  its  ending  at  the  apex  of  the  root.  In  this  tooth,  as 
well  as  in  all  those  previously  described,  the  apical  foramen  at 
the  time  of  completion  of  root-calcification  is  comparatively 
large,  and  readily  penetrated  during  operations  upon  it.  Figure 
151  illustrates  a  number  of  sections  of  an  upper  second  bicuspid 
at  maturity.  The  same  figure  also  represents  two  transverse 
sections,  A  being  at  the  cervical  line,  B  midway  between  the 
cervical  line  and  the  apex  of  the  root. 


PULP  CAVITIES  OF  'IHE  UPPER  MOLARS. 
The  inner  anatomy  of  the  molar  teeth  being  much  more  com- 
plicated than  any  of  those  previously  described,  it  will  be  found 
necessary  to  make  a  number  of  dissections  in  various  directions 
in  order  to  obtain  a  comprehensive  idea  of  the  location  and 
form  of  the  different  parts  of  the  pulp-cavity.  The  line  of 
demarcation  between  the  pulp-chamber  and  canals  is  always 
definite,  the  former  occupying  a  central  position  in  the  crown 
and  seldom  extending  beyond  the  cervical  line,  while  the  latter 
are  given  off  from  the  floor  of  the  chamber  and  penetrate  the 
various  roots,  their  entrances  being  marked  by  small  funnel- 
shaped  openings  in  the  floor  of  the  chamber.  In  the  matured 
tooth  the  form  of  the  chamber  usually  corresponds  to  that  of  the 
crown  of  the  tooth.  The  lateral  walls  of  the  chamber  are  four  in 
number,  and  are  named  according  to  their  location — mesial, 
distal,  buccal,  and  lingual.  The  average  thickness  of  these  walls 
at  maturity  is  about  equal  to  the  diameter  of  the  pulp-chamber. 
In  that  type  of  tooth  common  to  the  lymphatic  temperament 
where  there  is  but  little  constriction  at  the  neck,  resulting  in  the 
various  sides  of  the  tooth-crown  being  nearly  parallel  with  each 
other,  the  pulp-chamber  is  nearly  quadrilateral  in  form  ;  but  in 
those  teeth  marked  by  a  decided  constriction  at  the  neck,  most 
marked  in  the  nervous  temperament,  the  extent  of  surface  cov- 


ered  by  the  floor  of  the  chamber  is  much  less  than  that  occupied 
by  the  occlusal  portion.  In  the  former  class,  the  entrances  to 
the  various  canals  are  much  farther  apart  than  in  the  latter. 
The  occluding  wall  is  usually  much  thicker  than  the  lateral 
walls,  and  is  penetrated  by  the  horns  of  the  pulp-chamber,  one 
of  which  extends  into  each  cusp.  As  in  the  biscuspids,  the  extent 
to  which  the  horns  penetrate  the  cusps  is  controlled  by  the 
prominence  of  the  latter.  The  floor  of  the  pulp-chamber  is 
irregularly  rounded,  being  high  in  the  corner  and  gradually 
falling  away  in  the  direction  of  the  canals.  The  entrances  to 
the  root-canals,  three  in  number,  are  placed  in  the  form  of 
an  irregular  triangle,  called  the  molar  triangle.  The  mesial 
side  of  the  triangle  is  usually  the  longest,   the  distal  next  in 

length, and  the  buccal  theshortest. 
A  In  young  teeth  the  entrances  to 

^  the  canals  are  usually  in  the  form 
of  funnel-shaped  openings,  are 
comparatively  easy  of  access,  but 
^  after  maturity  may  disappear  and 
be  but  little  larger  than  the  canals 
themselves.  To  properly  study 
the  position  occupied  by  the  en- 
trance to  the  canals  on  the  floor 
of  the  pulp-chamber,  a  transverse 
section  of  the  tooth  shouldbemade 
at  a  point  somewhat  above  the  cer- 
vical line,  at  the  same  timepreservingboth  the  crown  and  the  roots 
of  the  tooth  for  comparison.  The  entrance  to  the  lingual  canal, 
which  is  usually  the  largest  and  most  readily  accessible,  may  be 
located  by  a  line  drawn  through  the  center  of  the  occlusal  sur- 
face of  the  crown  (Fig.  152)  from  buccal  to  lingual,  A,  and  by 
another  line  drawn  from  mesial  to  distal  almost  parallel  with  the 
linguomarginal  ridge,  passing  through  the  summits  of  the  mesio- 
lingual  and  distolingual  cusps,  B  ;  the  point  at  which  these  two 
lines  intersect  will  mark  the  approximate  location  of  the  lingual 
canal.  The  entrance  to  the  mesiobuccal  canal  may  be  located 
by  a  line  drawn  from  the  inner  side  of  the  mesiobuccal  angle  to 
a  corresponding  position   near  the  distobuccal  angle,  C.     This 


Fic.   152. 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  155 

should  be  intersected  by  a  line  drawn  from  the  summit  of  the 
mesiobuccal  cusp  to  the  summit  of  the  mesiolingual  cusp,  D, 
the  point  at  which  these  two  lines  cross  marking  the  entrance 
to  the  mesiobuccal  canal.  The  location  of  the  entrance  to  the 
distobuccal  canal  is  found  by  the  line,  C,  which  is  intersected 
by  another  line,  E,  drawn  from  the  summit  of  the  distobuccal 
cusp  to  a  corresponding  point  on  the  distolingual  cusp.  The 
nearer  the  tooth-crown  approaches  to  the  quadrilateral,  the 
nearer  will  the  molar  triangle  approach  the  equilateral. 


Fig.    153. — PuLP-CAViTY   of  Upper    First   Molar,   from   the   Fifth   to   the   Ninth 
Year.     Lingual  and  Mesiobuccal  Canals. 


Upper  First  Molar. — In  the  dissection  of  this  tooth,  the 
pulp-chamber  and  two  of  the  root-canals  only  can  be  shown, 
but  these  will  be  sufficient  to  pursue  the  study  with  intelligence. 
Figure  153  shows  a  number  of  longitudinal  sections,  made  in 
such  a  manner  as  to  expose  the  lingual  canal,  usually  the 
largest,  and  the  mesiobuccal  canal.  No.  i  illustrates  the 
approximate  size  and  form  of  the  pulp-chamber  at  the  fifth  year. 
At  this  period  the  chamber  occupies  a  large  proportion  of  the 
center  of  the  tooth-crown.  Two  of  the  four  horns  are  seen,  one 
of  which  penetrates  the  mesiobuccal  cusp,  and  one  the  mesio- 
lingual cusp.  In  many  instances  the  horns  of  the  molar  teeth 
are  quite  slender,  penetrating  the  dentin  to  a  greater  depth 
than  that  shown  in  the  illustration,  in  the  form  of  minute  hair- 
like projections,  which  in  some  instances  reach  almost  to  the 
enamel  walls. 

No.  2  illustrates  the  condition  of  the  pulp-cavity  at  the  sixth 
year,   or  at  the   time  of   eruption.     The  outline    of    the    pulp- 


256  ANATOMY. 

chamber  is  completed,  and  die  floor  has  begun  to  make  its 
appearance  by  a  central  deposit  of  dentin.  It  will  be  observed 
that  the  lateral  walls  of  the  chamber  are  somewhat  less  in  thick- 
ness than  the  occluding  wall,  a  condition  which  will  become  more 
pronounced  as  the  tooth  develops.  With  the  beginning  of  the 
formative  process  in  the  floor  of  the  chamber  we  find  the 
trifurcation  of  the  roots  established,  and  the  beginning  of  the 
canals  outlined.  The  canals  at  this  period  are  quite  similar  to 
those  of  the  bicuspid,  being  provided  with  a  funnel-shaped 
extremity,  which  extends  from  the  free  calcifying  margins  of  the 
roots  to  the  floor  of  the  chamber.  No.  3  shows  the  change 
which  has  taken  place  at  the  seventh  year.  While  the  pulp- 
chamber  is  somewhat  reduced  in  size,  but  little  change  is  notice- 
able in  its  outline.  By  this  time  the  floor  of  the  chamber  has 
become  an  important  factor  in  the  tooth  development.  By  the 
constant  lateral  extension  of  this  central  deposit  of  dentin  the 
floor  of  the  chamber  is  gradually  spread  out,  this  alteration 
being  at  the  expense  of  the  entrances  to  the  root-canals,  which 
become  reduced  in  diameter  as  the  floor  is  extended.  The  horns 
of  the  chamber  are  slightly  less  prominent,  but  this  part  ot 
the  cavity  has  the  appearance  of  having  receded  bodily  root- 
ward.  The  roots  have  advanced  somewhat  beyond  the  point  of 
trifurcation,  and  a  definite  outline  has  been  given  to  the  canals. 
At  this  period  the  diameter  of  the  root-wall  is  about  equal  to  the 
diameter  of  the  pulp-canal.  Along  with  the  gradual  decrease  in 
the  diameter  of  the  roots,  there  is  observed  a  corresponding 
decrease  in  the  width  of  the  funnel-shaped  extremities  of  the 
canals. 

At  the  eighth  year.  No.  4,  a  gradual  reduction  in  the  capa- 
city of  both  the  chamber  and  canal  is  noted.  Accompany- 
ing the  above  condition  there  is  found  a  corresponding  in- 
crease in  the  thickness  of  the  surrounding  walls.  The  horns 
of  the  pulp-chamber  are  much  reduced  in  size,  and  the  form  of 
the  chamber  more  closely  resembles  that  of  the  general  contour 
of  the  tooth-crown.  The  increase  in  the  length  of  the  roots  is 
proportionately  greater  than  that  of  previous  years,  in  conse- 
quence of  which  the  length  of  the  canals  is  increased  to  a  greater 
degree.     In  No.  5  the  maximum  size  of  the  chamber  and  canals 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  257 

is  apparent,  which  condition  takes  place  about  the  ninth  year, 
or  at  a  time  when  calcification  of  the  tooth  is  completed  ex- 
ternally. In  some  instances,  owing  to  the  additional  length 
of  the  lingual  root,  the  apical  foramen  may  not  be  established 
before  the  tenth  year.  At  this  latter  period  it  is  safe  to  assume 
that  all  three  canals  have  completed  their  longitudinal  extent, 
and  the  foramina,  although  proportionately  large,  have  been 
established,  so  that  a  more  definite  description  of  each  canal 
may  be  given.     ^\\&  lingual  canal  (Fig.    154,  A)  is  usually  the 


Fig.  154. — PuLP-CAViTiEs  of  Upper  First  Molar,  Slightly  Enlarged. 


largest,  and  branches  off  from  the  floor  of  the  chamber,  near  the 
mesiodistal  center  of  the  extreme  lingual  margin,  the  entrance 
in  the  average  tooth  being  well  defined  by  a  circular,  funnel- 
shaped  opening.  The  direction  of  this  canal  is  usually  upward 
and  slightly  inward,  until  the  apical  extremity  is  approached,  at 
which  point  it  is  inclined  to  the  buccal.  The  circular  form  pre- 
sented at  the  beginning  of  the  canal  is  generally  continued 
throughout  its  entire  length,  in  this  respect  differing  from  the 
two  buccal  canals.  The  average  length  of  the  lingual  canal  is 
about  Yi.  of  an  inch.  The  viesiobuccal  canal  (Fig.  1 54,  B) 
17 


258  ANATOMY. 

branches  off  from  the  floor  of  the  chamber,  at  its  extreme  mesio- 
buccal  angle,  and  the  entrance,  instead  of  being  funnel-shaped 
and  easy  of  access,  is  flattened  from  mesial  to  distal,  and  fre- 
quently difficult  to  enter.  This  flattened  form  continues  through- 
out its  course,  which  for  the  distance  of  ^  of  an  inch  is  in  a 
buccal  and  mesial  direction  ;  beyond  this  point  it  is  usually  in- 
clined to  the  buccal,  until  the  upper  third  of  the  root  is  reached, 
where  it  turns  rather  abruptly  to  the  distal.  This  canal  is  gen- 
erally a  trifle  shorter  than  the  lingual,  averaging  about  ^  to  yV  of 
an  inch.  The  distobuccal  ozxi-aX  (Fig.  154,  C)  branches  off  from 
the  floor  of  the  chamber  at  the  extreme  distobuccal  angle.  In 
those  teeth  which  most  nearly  approach  the  quadrilateral  form, 
the  entrance  to  this  canal  will  be  farther  from  the  center  of  the 
tooth,  the  molar  triangle  in  this  instance  being  almost  an  equila- 
teral. It  sometimes  happens  that  the  entrance  to  this  canal  is  di- 
rectly in  the  floor  of  the  pulp-chamber,  near  to,  but  not  against, 
its  buccodistal  angle.  The  entrance  is  usually  abrupt,  seldom 
being  funnel-shaped,  making  it  by  far  the  most  difficult  of 
access.  It  is  inclined  to  be  circular  in  form,  and  more  or  less 
tortuous  in  its  course.  Immediately  above  the  point  of  begin- 
ning it  is  inclined  toward  the  buccal  and  distal ;  near  its  center 
it  may  incline  slightly  to  the  mesial ;  and  finally,  at  its  upper 
third,  turns  somewhat  abruptly  in  a  distobuccal  direction.  This 
canal  is  usually  the  shortest  of  the  three,  its  average  length 
being  about  ^g  of  an  inch. 

Figure  154  also  illustrates  a  number  of  transverse  sections  of 
this  tooth,  D  being  made  at  the  cervical  line,  looking  toward 
the  crown,  E  looking  toward  the  roots,  while  F  represents 
a  transverse  section  at  a  point  immediately  above  the  floor  of 
the  pulp-chamber. 

Upper  Second  Molar.  —  In  many  respects  the  pulp- 
chamber  of  this  tooth  is  similar  to  that  of  the  first  molar,  but 
there  are  a  few  variations  which  must  be  briefly  described. 
First,  the  outline  of  the  tooth-crown  being  much  more  flattened 
from  mesial  to  distal,  a  corresponding  variation  is  noted  in  the 
form  of  the  pulp-chamber,  increasing  the  length  of  the  mesial 
side  of  the  molar  triangle,  and  decreasing  the  length  of  the 
buccal  and  distal  sides.     The  chamber  is  more  or  less  flattened 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  259 

from  mesial  to  distal,  making  it  somewhat  oblong  from  buccal 
to  lingual.  Second,  on  account  of  a  reduction  in  the  promi- 
nence of  the  cusps,  the  horns  of  the  cavity  are  usually 
somewhat  less  pronounced  than  those  of  the  first  molar. 
Third,  the  floor  of  the  cavity  is  less  convex,  and  slopes  more 
gradually  toward  the  entrances  of  the  various  canals.  In  a 
general  way,  the  rules  given  for  ascertaining  the  approximate 
location  of  the  entrances  to  the  canals  in  the  first  molar  apply 
to  this  tooth.  The  comparative  size  and  form  of  the  pulp- 
chamber  and  canals  during  the  development  of  the  tooth  are 
shown  in  figure  155,  extending  from  the  ninth  to  the  sixteenth 
or  eighteenth  year,  at  which  latter  period  the  crown  and  roots 
of  the  tooth  are  fully  calcified  externally. 


9th  year  iithyear  13th  year  islh  year  l8th  year 

Fig.   155. — PuLP-CAviTiEs   in   the  Upper  Second  Molar,  from   the  Ninth   to  the 
Eighteenth  Year. 

Upper  Third  Molar. — In  this  tooth  the  conditions  are  so 
variable  that  a  description  of  the  pulp-cavity  taken  from  a  single 
tooth  would  be  insufficient.  In  the  majority  of  instances  the 
outline  of  the  tooth-crown  approaches  the  triangular  form,  and 
in  consequence  the  pulp-chamber  is  triangular  rather  than 
quadrilateral  or  oblong.  The  mesial  border  of  the  chamber  Is 
the  longest,  the  distal  next  in  length,  and  the  buccal  the  shortest 
of  the  three.  The  horns  are  generally  less  in  number  and  much 
less  pronounced  than  those  of  either  the  first  or  second  molars. 
The  floor  of  the  chamber  may  be  broken  by  irregularities  simi- 
lar to  those  previously  described,  or  it  may  be  entirely  absent, 
this  latter  condition  occurring  when  the  tooth  has  but  a  single 
root  accompanied  by  a  single  canal.  The  various  stages  of 
development  having  been  given  in  connection  with  the  general 


description  of  the  tooth,  no  attempt  will  be  made  to  describe  this 
by  longitudinal  sections,  the  complications  in  root-form  making 
such  a  proceeding  impracdcable.  Instead  of  so  doing,  the  space 
will  be  devoted  to  a  brief  description  of  the  variety  of  pulp- 
canals  found  in  this  tooth.  Probably  the  most  frequent  condition 
is  that  which  resembles  the  first  and  second  molars — i.  e.,  three 
canals  branching  off  from  the  chamber  in  as  many  different 
roots,  two  to  the  buccal  and  one  to  the  lingual.  When  the  three 
canals  exist,  the  entrances  to  them  will  be  well  beyond  the 
cervical  line,  where  they  will  be  found  clustered  much  closer 
together  than  those  of  the  first  and  second  molars,  this  difference 
in  their  location  being  so  marked  that  the  diagram  previously 
given  can  not  be  depended  upon  in  an  attempt  to  locate  them. 


Fig.   156. — Longitudinal  Sections,  Ui'per  Third  Molar,  Slightly  Enlarged. 

The  usual  course  of  these  canals  is  first  slightly  mesial,  then 
distal,  and  finally  in  a  distolingual  direction.  On  account  of 
the  pulp-chamber  e.xtending  well  beyond  the  cervical  line,  the 
canals  are  much  shorter  than  those  of  the  first  or  second 
molars,  their  average  length  being  less  than  yi  of  an  inch. 
Another  form  frequently  met  with  is  that  of  the  flattened 
single  canal,  occurring  when  the  tooth  has  but  a  single 
root,  which  shows  no  signs  of  trifurcating  (Fig.  156,  A).  In 
this  instance  the  pulp-chamber  gradually  passes  into  the  canal, 
and  the  chamber  is  without  a  floor.  Such  a  canal  is  shaped 
like  the  chamber  at  its  point  of  beginning ;  but  as  it  passes 
toward  the  apex  it  becomes  flattened  in  the  direcdon  of  the 
smallest  diameter  of  the  root.     But  litde  difficulty  is  experienced 


THE  PULP-CAVITIES  OF  THE  UPPER  TEETH.  261 

in  entering  such  a  canal,  and  usually  it  is  readily  followed  to  its 
apex.  Another  condition  frequently  met  with  in  the  single- 
rooted  third  molar  is  that  of  one  or  more  canals  branching  off 
from  the  floor  of  the  chamber,  their  course  through  the  root- 
substance  being  without  regard  to  the  external  contour  of  the 
root  (Fig.  156,  B).  These  canals,  which  may  exist  to  the  num- 
ber of  five  or  six,  are  usually  very  minute,  and  in  some  instances 
may  pass  from  the  floor  of  the  chamber  to  the  apex  of  the  root 
almost  in  a  direct  line,  and  end  in  distinct  foramina,  or  they  may 
take  a  tortuous  course,  and  when  near  the  apex  unite,  ending  in 
a  single  foramen.  When  the  tooth  is  provided  with  four,  five,  or 
even  six  small  roots,  as  sometimes  occurs,  each  root  will  be 
traversed  by  a  minute  canal,  the  entrances  to  these  being 
variously  placed  about  the  floor  and  lateral  margins  of  the 
pulp-chamber  (Fig.  156,  C).  In  all  operations  upon  this  tooth 
it  must  be  recalled  that  it  is  the  last  to  be  calcified,  and  conse- 
quently the  canals  and  foramina  are  proportionately  larger  than 
in  the  other  teeth  ;  at  the  same  time,  it  possesses  one  advantage 
over  the  others — i.  e.  (with  the  single  exception  of  the  cuspid), 
being  fully  calcified  at  or  about  the  time  of  its  eruption. 


ANATOMY. 


PULP-CAVITIES  OF  THE  LOWER  TEETH. 


Fig.  157. — PuLP-CAViTiES  of  the  Lower  Incisor. 

The  outline  of  the  pulp-cavities  of  the  lower  teeth,  like  those 
of  the  upper,  corresponds  to  the  general  tooth  contour.  The 
comparative  size  of  the  cavity  at  various  stages  of  tooth  develop- 
ment will  not  be  repeated  in  this  description,  the  conditions 
being  similar  to  those  in  the  upper  teeth  (see  also  Develop- 
ment of  the  Teeth). 

Lower  Incisors. — The  pulp  cavities  of  the  lower  central 
and  lateral  incisors  are  so  nearly  alike  that  a  single  description 
will  answer  for  both.  Figure  156,  A,  represents  a  labiolingual 
section  of  a  lower  incisor,  showing  the  most  frequent  form  of 
the  pulp-cavity.  The  tooth  from  which  the  section  was  prepared 
was  one  about  middle  life,  the  cavity  in  younger  teeth  being 
proportionately  larger,  while  a  gradual  decrease  in  diameter 
would  be  noted  with  advancing  age.  There  is  no  mark  of  dis- 
tinction between  the  pulp-chamber  and  canal,  so  that  an  imagin- 
ary separation  would  have  to  be  made  at  the  cervical  line,  or 
slightly  below  that  point.  Taken  In  its  entirety,  the  cavity 
presents  the  form  of  a  double  cone,  the  common  base  of  which 
is  slightly  to  rootward  of  the  cervical  line.  The  chamber 
penetrates  the  crown  fully  half-way  to  the  cutting-edge,  at 
which  point  it  ends  in  a  thin,  fan-like  margin  (best  observed  in 


PULP-CAVITIES   OF  THE   LOWER  TEETH.  263 

mesiodistal  section),  while  the  canal  gradually  decreases  in 
size  until  the  apical  foramen  is  reached.  Although  this  is  the 
most  common  form  of  the  pulp-cavity  in  the  lower  incisors,  it 
is  by  no  means  the  constant  condition.  The  tooth  is  not  infre- 
quently provided  with  a  medium-sized  pulp-chamber,  which 
extends  somewhat  below  the  cervical  line,  beyond  which  point  it 
branches  into  two  fine  canals,  which  are  continued  separately 
until  the  apical  third  of  the  root  is  approached,  when  they  again 
unite,  and  finally  end  in  a  single  foramen.  Figure  157,  C, 
represents  a  mesiodistal  section  of  a  young  lower  incisor,  in 
which  the  three  small  horns  of  the  pulp-chamber  are  apparent. 
At  this  period  the  fan-shaped  extremity  of  the  pulp-chamber 
occupies  about  one-half  of  the  mesiodistal  diameter  of  the  crown, 
and  the  horn-like  projections  extend  well  toward  the  enamel  cap. 
Figure  157,  B,  shows  the  average  size  and  form  of  the  pulp- 
cavity  at  maturity,  by  a  mesiodistal  section  through  the  long  axis 
of  the  tooth.  In  this  it  will  be  observed  that  the  horns  of  the 
pulp-cavity  have  disappeared,  and  that  the  capacity  of  the 
cavity  in  general  is  much  reduced. 

Lower  Cuspids. —  The  pulp-chamber  and  canal  in  this 
tooth,  while  usually  conforming  to  the  general  contour  of  the 
tooth,  are  frequently  found  to  vary  greatly,  both  in  outline  and  in 
size.  The  most  common  form,  however,  is  that  shown  in  figure 
158,  A,  a  labiolingual  section  of  an  adult  tooth.  The  chamber 
and  canal  have  no  line  of  demarcation,  and  unite  at  a  common 
base  considerably  below  the  cervical  line,  the  former  penetrating 
the  crown  of  the  tooth  to  a  point  about  midway  between  the 
cervical  line  and  the  summit  of  the  cusp,  at  which  point  it  ends 
in  a  sharp,  hair-like  projection.  Accompanying  this  common 
form  there  is  much  variation  in  size,  even  in  teeth  of  the  same 
age.  Figure  158,  B,  shows  another  labiolingual  section  of  an 
adult  lower  cuspid,  in  which  the  pulp-cavity  fails  to  accurately 
follow  the  outline  of  the  tooth,  and  its  capacity  is  much  less 
than  that  shown  at  A.  The  root  of  this  tooth  is  in  most 
instances  circular,  in  which  case  the  canal  will  be  similarly 
formed ;  but  occasionally  the  root  will  bg  much  flattened  Irom 
mesial  to  distal,  and  as  a  result  of  this  the  canal  will  also  be 
much  flattened.     The  canal  of  this  tooth  is  seldom  divided.      In 


264 


figure  158,  C,  a  mesiodistal  section  of  a  lower  cuspid  is 
shown,  and  it  will  be  observed  that  the  fan-shaped  extremity 
of  the  chamber  common  to  the  incisors  is  absent,  the  cavity 


Fig.  158. — Pulp-cavities  of  the  Lower  Cuspids. 

ending  rather  abruptly,  or  by  a  fine  line  near  the  center  of  the 
crown.  The  same  illustration  also  shows  a  number  of  trans- 
verse sections,  which  will  give  an  idea  of  the  form  of  the  cavity 
at  various  parts  of  the  tooth. 

Lower  Bicuspids. — The  pulp-cavities  of  these  teeth  may 
be  best  described  collectively,  thus  affording  an  opportunity  for 
comparison.  Unlike  the  upper  bicuspids,  it  is  seldom  that 
the  canals  are  definitely  separated  from  the  chambers.  That 
part  of  the  cavity  within  the  crown,  however,  is  usually  quite 
wide  from  buccal  to  lingual,  and  unites  with  the  canal  by  a  long, 
funnel-shaped  constriction.  The  center  of  the  pulp-chamber 
may  be  considered  as  being  about  on  a  level  with  the  cervical 
line.  In  the  first  bicuspid  the  pulp-cavity  is  provided  with  a 
single  horn,  which  extends  with  more  or  less  prominence  in  the 
direction  of  the  buccal  cusp.  That  part  of  the  chamber  facing 
the  lingual  cusp  is  usually  rounded  off.      In  the  second  bicuspid 


PULP-CAVITIES   OF  THE    LOWER  TEETH. 


265 


the  occlusal  wall  of  the  pulp-chamber  generally  presents  a  dif- 
ferent form  ;  two  well-defined  horns  are  usually  present,  of  which 
the  buccal  is  the  longest ;  or  the  chamber  may  be  prominently 
rounded  at  these  points.  The  pulp-chamber  of  the  second 
bicuspid  is  generally  larger  than  that  of  the  first.  The  canals 
of  these  teeth  are  usually  circular  throughout,  and  are  readily 
penetrated  until  the  apical  third  is  reached,  beyond  which  point 
they  are  extremely  small.  In  some  instances  the  canal  divides 
near  the  center  of  the  root,  and  is  continued  as  two  canals,  end- 
ing in  distinct  foramina,  or,  after  separating,  they  may  again 
unite,   and  end  in  a  single  foramen.      In  figure    159    the   aver- 


FiG.  159. — Sections  of  Lower  Bicuspids. 


age  size  and  form   of  the  canal  in  these  teeth  is  shown  by  a 
number  of  mesiodistal  and  transverse  sections. 

Lower  Molars. — The  form  of  the  pulp-chambers  of  the 
lower  molars  corresponds  to  the  general  outline  of  the  crown, 
and  the  form  of  the  root-canals  is  similar  to  the  general  contour 
of  the  roots.  The  pulp-chambers  approach  the  quadrilateral 
form  ;  the  buccal  and  lingual  sides  are  somewhat  the  longest,  the 
mesial  next  in  length,  and  the  distal,  usually  slightly  rounded, 
is  the  shortest.  The  occluding  wall  is  convex  rootward,  sloping 
in  the  direction  of  the  various  cusps,  each  of  which  is  penetrated 
by  a  horn.      Like  the  horns  of  the  pulp-chambers  in  general,  the 


extent  to  which  these  penetrate  the  cusps  is  influenced  by  the 
age,  type,  and  functional  activity  of  the  organ.  The  floor  of  the 
cavity  is  convex  in  the  direction  of  the  occlusal  surface,  but  this 
convexity  is  principally  from  mesial  to  distal.  From  the  summit 
of  this  convexity  the  floor  slopes  to  the  entrances  of  the  canals, 
the  opening  into  which  is  inclined  to  be  funnel-shaped  rather 
than  abrupt.  The  lateral  walls  of  the  chamber  are  much  inclined 
to  follow  the  general  contour  of  the  crown.  The  horns  of  the 
pulp-chamber  are  usually  more  pronounced  in  the  first  than  in  the 
second  molar,  and  still  less  clearly  defined  in  the  third  than  in 


Fig.  i6o. — Sections  ok  Lower  Molars,  Enlarged  about  One-third. 


the  second.  The  roots  of  the  first  molar  being  somewhat  further 
apart  than  those  of  the  second,  the  floor  of  the  chamber  in  the 
former  is  slightly  more  extensive  than  in  the  latter.  To  study 
the  pulp-cavities  of  these  teeth  a  longitudinal  section  should  be 
made  through  the  center  of  the  tooth  from  mesial  to  distal. 
Figure  i6o.  A,  shows  such  a  dissection  through  the  first  molar, 
and  illustrates  the  average  size  and  form  of  the  chamber  and 
canals  at  adult  age.  The  canals  join  the  chamber  by  a  funnel- 
shaped  opening,  and  but  little  difficulty  will  be  found  in  effecting 
an  entrance,  but  to  follow  them  to  their  apices  will  be  more  per- 


PULP-CAVITIES    OF  THE   LOWER  TEETH.  267 

plexing.  The  roots  of  this  tooth  being  much  flattened  from 
mesial  to  distal,  the  canals  are  also  flattened  in  this  direction, 
but  broad  from  buccal  to  lingual.  The  tntrances  to  these  canals 
may  be  found  at  the  extreme  mesial  and  distal  margins  of  the 
pulp-chamber,  and  usually  extend  from  the  buccal  to  the  lingual 
walls  of  the  cavity.  It  is  not  uncommon  for  the  mesial  canal  to 
divide  soon  after  leaving  the  chamber,  and  continue  as  two 
canals,  ending  in  separate  foramina  (Fig.  i6o,  B).  This  con- 
dition is  seldom  present  in  the  distal  canal,  which  is  usually 
straight  from  its  mouth  to  the  apical  foramen.  The  capacity  of 
the  pulp-chamber  is  usually  a  trifle  less  than  that  of  the  first 
molar,  and  the  entrances  to  the  canals  are  somewhat  nearer 
together.  In  other  respects  the  cavity  is  similar  to  that  of  the 
first  molar.  Figure  160  also  shows  a  number  of  transverse 
sections  through  a  lower  molar,  and  gives  an  idea  of  the  size 
of  the  canals  at  various  parts  of  the  roots.  In  some  instances 
the  roots  of  the  second  molar  coalesce,  in  which  case  a  single 
root-canal  may  be  present.  In  the  third  molar  the  most  com- 
mon form  of  the  pulp-cavity  is  one  similar  to  that  of  the  first, 
but  both  the  chamber  and  canals  are  smaller.  Unlike  the  pulp- 
cavity  of  the  corresponding  upper  tooth,  this  tooth  is  not  sub- 
ject to  so  much  variation,  although  it  is  sometimes  found  with  a 
single  root  traversed  by  a  single  canal,  which  may  be  accom- 
panied by  a  rather  large  pulp-chamber. 


CHAPTER  XI. 

THE  DECIDUOUS  TEETH,  THEIR  ARRANGEMENT,  OCCLUSION,  ETC; 
THEIR  CALCIFICATION,  ERUPTION,  DECALCIFICATION,  SHEDDING 
PROCESS,  AND  AVERAGE  MEASUREMENTS;  THEIR  SURFACES, 
GROOVES,  FOSS^,  RIDGES,  SULCI,  AND  PULP-CAVITIES. 

THE  DECIDUOUS  TEETH. 

Central  Incisor  Lateral  Incisor  Cuspid  First  Molar  Second  Molar 


Fig.    i6i. — The   Deciduous  Teeth,  Upper   and   Lower,  from  the  Left  Side  of 
THE  Mouth. 


As  implied  by  the  term  deciduous,  these  teeth  are  temporary 
in  their  nature,  and,  after  subserving  the  purposes  of  early 
childhood,  are  thrown  off  by  an  operation   of  the  economy  to 


THE  DECIDUOUS  TEETH.  269 

give  place  to  the  permanent  organs.  The  shedding  process 
takes  place  in  the  incisors  between  the  seventh  and  eighth  years, 
in  the  molars  from  the  tenth  to  the  eleventh  years,  and  in  the 
cuspids  about  the  twelfth  year.  This  shedding  process,  however, 
does  not  indicate  the  period  at  which  the  degeneracy  of  the 
tooth  begins,  for,  in  a  year  or  two  after  the  roots  are  completely 
formed  and  the  apical  foramen  established,  decalcification  begins 
at  the  apical  ends  and  continues  in  the  direction  of  the  crown 
until  absorption  of  the  entire  root  has  taken  place  and  the 
crown  is  lost  from  lack  of  support.  Decalcification  in  the 
incisors  begins  between  the  fourth  and  fifth  years,  in  the  molars 
from  the  seventh  to  the  eighth  years,  and  in  the  cuspids  about 
the  ninth  year. 

The  deciduous  teeth  are  twenty  in  number,  ten  in  each  jaw, 
and  may  be  classified  as  follows :  Fonr  incisors,  tzvo  cuspids, 
and  four  molars.  The  incisors,  central  and  lateral,  occupy 
the  central  portion  of  the  arch,  are  placed  two  upon  each  side 
of  the  median  line,  and  are  succeeded  by  the  four  permanent 
incisors,  which  finally  occupy  the  same  position.  The  cuspids 
are  located  immediately  to  the  distal  of  the  lateral  incisors,  and 
are  displaced  by  the  permanent  cuspids.  The  first  and  second 
molars  come  next  in  the  arch,  but,  unlike  the  anterior  teeth,  are 
followed  by  permanent  successors  of  another  class,  the  first  and 
second  bicuspids,  the  permanent  molars  erupting  posteriorly  to 
these  as  the  jaw  increases  in  length. 

In  general  the  deciduous  teeth  resemble  their  permanent 
successors,  yet  there  are  a  number  of  minor  differences  which 
will  require  a  comparative  description.  Both  the  crowns  and 
the  roots  are  much  smaller  in  every  direction  than  those  of  the 
permanent  teeth,  but  the  diameter  of  the  crowns  is  propor- 
tionately greater  than  that  of  the  roots,  while  the  roots  are 
proportionately  longer.  The  fact  that  the  roots  are  smaller  in 
proportion  than  the  crowns  is  productive  of  a  neck  much  more 
constricted.  The  roots  of  the  deciduous  teeth  are  the  same  in 
number  as  those  of  the  corresponding  permanent  teeth,  the 
incisors  and  cuspids  being  provided  with  one,  the  upper  molars 
with  three,  and  the  lower  molars  with  two. 


THE  OCCLUSION   OF  THE   DECIDUOUS  TEETH. 


First  Molar 
Second  Molar 

Permanent  First 


Cuspid        Incisors 


Fig.  162. — The  Upper  Dental  Arch  about  the  Seventh  Year. 


The  arranjrement  of  the  deciduous  teeth  in  the  jaws  is  similar 
to  that  of  the  permanent  organs,  the  upper  teeth  describing" 
the  segment  of  a  larger  circle  than  the  lower,  in  consequence 
of  which  the  upper  teeth  close  over  or  outside  of  the  lower. 
The  character  of  the  occlusion  in  the  deciduous  teeth  is  not 
subject  to  so  much  variation  as  that  found  in  connection  with 
the  permanent  set,  this  being  accounted  for  by  the  more 
constant  form  in  the  crowns  of  the  former.  The  relations 
existing  between  the  upper  and  lower  deciduous  teeth 
when  in  contact  is  such  that  each  tooth,  with  the  exception 
of  the  lower  central  incisor  and  the  upper  second  molar, 
occludes  with  two  teeth  of  the  opposite  jaw,  the  upper 
central  incisor  being  opposed  by  the  entire  cutting-edge  of  the 
lower  central  and  the  mesial  third  of  the  lower  lateral  ;  the 
upper  lateral  coming  in  contact  with  the  remaining  two-thirds 
of  the  lower  lateral  and  a  portion  of  the  mesial  half  of  the 
lower  cuspid,  this  arrangement  continuing  throughout  the 
series.  The  foregoing  description  of  the  occlusion  of  the  de- 
ciduous teeth  is  applicable  to  but  a  small  part  of  their  transi- 
tory existence.      By  the  time  they  are  fully  erupted  and  have 


THE  DECIDUOUS  TEETH  IN    DETAIL.  271 

assumed  their  respective  positions  in  the  arch,  the  increase 
in  the  size  of  the  bone  is  sufficient  to  create  a  slight  space 
between  the  teeth,  which  condition  is  soon  followed  by  a 
greater  separation  through  the  protrusion  of  the  anterior  teeth, 
caused  by  the  growth  and  approach  of  the  permanent  teeth 
from  behind. 

The  calcification  of  the  deciduous  teeth  is  similar  to  that  of 
the  permanent,  the  process  in  the  incisors  and  cuspids  begin- 
ning along  the  cutting-edges  in  three  distinct  lobes,  while  in  the 
molars  a  center  of  calcification  is  provided  for  each  cusp  (see 
Development  of  the  Teeth). 


THE  DECIDUOUS  TEETH  IN   DETAIL. 


Fourth  Year 


Fortieth  Week 


Twentieth  Week 


Seventh  Year 


Fig.  163. 


UPPER  CENTRAL  INCISOR. 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Seventeenth  to  Eighteenth  Month  after  Birth- 
Erupts,  Sixth  to  Eighth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fourth  Year. 

Shedding  Process  Takes  Place,  about  the  Seventh  Year. 
Average  Length  of  Crown,  .23. 

Average  Length  of  Root,  .39. 

Average  Length  over  All,  .62. 


This  tooth,  as  well  as  all  of  the  deciduous  teeth,  presents  for 
examination  numerous  surfaces,  margins,  and  angles,  these  being 
the  same  in  name  and  location  as  those  of  the  permanent  teeth. 


The  Labial  Surface  of  the  Crown  (Fig.  164). — Tliis  sur- 
face is  smooth  and  generally  convex,  but  with  an  inclination  to 
flatness  near  the  incisive  margin.  The  mesial  margin  is  slightly 
convex  in  the  direction  of  the  length  of  the  tooth,  and  rounded 
from  labial  to  lingual.  The  distal  margin  is  decidedly  convex 
from  the  cutting-edge  to  the  cervical  line,  in  many  instances 
forming  almost  a  complete  semicircle,  which  is  usually  at  the 
expense  of  the  distal  angle  of  the  crown.  The  cervical  margin 
is  deeply  concave  in  the  direction  of  the  root,  and  the  incisive 
margin  is  straight  over  its  central  portion  and  rounded  or  angu- 
lar at  its  extremities.  The  labial  grooves 
are  seldom  so  well  defined  as  those  upon  the 
permanent  incisors. 

The  Lingual  Surface  of  the  Crown. — 
In  some  instances  this  surface  is  smooth  and 
concave  near  the  cutting-edge  and  convex 
over  the  cervical  portion,  with  the  marginal 
ridges  well  defined.  In  other  cases  it  is  con- 
cave from  the  cutting-edge  to  the  cervical 
ridge  being  provided  with  a  longitudinal 
ridge  in  the  center,  a  slight  depression  upon 
eitherside,  and  marginal  ridges  poorlydefined. 
In  the  former  instance  the  lingual  fossa  is 
present ;  in  the  latter  it  is  absent.  The  mesial  and  distal  sur- 
faces of  the  crown  are  both  smooth  and  convex,  the  former 
being  inclined  to  flatness  over  its  cervical  third — a  condition 
which  is  seldom  present  in  the  latter.  The  mesial  angle  is  alone 
well  defined,  the  cutting-edge  passing  into  the  distal  surface  with 
a  long,  gradual  sweep,  thus  in  a  measure  destroying  the  distal 
angle.  The  neck  of  the  tooth  is  marked  by  a  decided  constric- 
tion, which  is  principally  produced  at  the  expense  of  the  crown 
alone.  The  root  of  the  tooth,  when  compared  with  the  root  of 
the  permanent  central  incisor,  is  much  longer  in  proportion  to 
the  length  of  the  crown.  In  some  instances  it  is  flattened  from 
mesial  to  distal,  these  two  sides  converging  as  they  pass  to  the 
lingual ;  in  others  it  is  flattened  from  labial  to  lingual.  Gener- 
ally speaking,  it  is  a  straight  root,  but  is  occasionally  provided 


THE  DECIDUOUS  TEETH    IN  DETAIL. 


273 


with  a  slight  mesial  curve  near  its  apical  third,  and  it  is  some- 
times curved  slightly  from  labial  to  lingual. 


UPPER  LATERAL  INCISOR. 


Kih  /  3 

Seventh  Year 


Eighth  Ye 


Fortieth  Week 
Twentieth  Week 


Fig.    165. 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Fourteenth  to  Sixteenth  Month  after  Birth. 
Erupts,  Seventh  to  Ninth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fifth  Year. 

Shedding  Process  Takes  Place,  about  the  Eighth  Year. 
Average  Length  of  Crown,  .25. 

Average  Length  of  Root,  .45. 

Average  Length  over  All,  .70. 

The  various  surfaces  of  this  tooth  so 
closely  resemble  those  of  the  central  incisor 
that  a  separate  description  will  be  unneces- 
sary ;  in  a  general  way,  however,  there  are 
a  few  minor  points  of  difference.  The  tooth 
is  smaller  in  every  direction  excepting  in 
its  length,  which  is  generally  equal  to  and 
frequently  greater  than  that  of  the  central 
incisor.  The  diameter  of  the  root  is  but 
little  less  than  that  of  the  central,  while 
the  mesiodistal  measurement  of  the  crown 
is  about  one-third  less,  in  consequence  of 
which  the  neck  of  the  tooth  is  not  so  well  defined.  The 
aneles  of  the  crown  are  more  rounded  than  those  of  the  central 


Fig.  166. — Upper  Lat- 
eral Incisor,  Labial 
Surface. 


ANATOMY. 


UPPER  CUSPID. 


Second  Year 


Fig.  167. 

Calcification  Begins,  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  about  Two  Years  after  Birth. 

Erupts,  Seventeenth  to  Eighteenth  Month  after  Birth. 
Decalcification  Begins,  about  the  Ninth  Year. 

Shedding  Process  Takes  Place,  about  the  Twelfth  Year. 
Average  Length  of  Crown,  .25. 

Average  Length  of  Root,  .53. 

Average  Length  over  All,  .78. 

Like  the  permanent  cuspid,  the  general  contour  of  this  tooth 
is  that  of  a  double  cone,  the  lines  of  which  are  somewhat  broken. 
The  greatest  mesiodistal  extent  of  the  crown 
is  from  angle  to  angle,  and  this  measurement 
about  corresponds  to  the  width  of  the  crown 
of  the  central  incisor. 

The  Labial  Surface  of  the  Crown 
(Fig.  168). — This  surface  is  strongly  convex 
from  mesial  to  distal,  and  slightly  so  from 
the  cutting-edge  to  the  cervical  line.  It  is 
bounded  by  five  margins  :  mesial,  distal,  cer- 
vical, mesio-incisive,  and  disto-incisive.  The 
mesial  and  distal  margins  are  rounded  and 
smooth,  the  cervical  well  outlined  by  the 
cervical  line  and  base  of  the  cervical  ridge, 
while  the  two  incisive  margins  are  formed  by  the  mesial  and 
distal  cutting-edges.  The  labial  grooves  are  thrown  well 
toward  the  lateral  margins,  and  are  usually  more  distinct  than 
those  upon  the  incisors.     The  labial  ridge  is  prominent. 


Fig.  168. — Upper  Cuspid, 
Labial  Surface. 


THE  DECIDUOUS  TEETH    IN  DETAIL.  275 

The  Lingual  Surface  of  the  Crown. — This  surface  is 
generally  divided  into  two  portions  by  the  lingual  ridge,  which 
extends  from  the  summit  of  the  cusp  to  the  base  of  the  cervical 
ridge.  On  either  side  of  this  ridge  are  the  lingual  grooves,  but 
which  appear  more  in  the  form  of  small  fossse.  The  marginal 
ridges  are  fairly  well  defined. 

The  Mesial  and  Distal  Surfaces  of  the  Crown. — The 
extent  of  these  two  surfaces  is  frequently  much  interfered  with 
by  the  slope  of  the  mesial  and  distal  cutting-edges,  which  may 
be  so  long  that  the  angles  of  the  crown  are  forced  well  toward 
the  cervical  line,  in  some  instances  almost  obliterating  these  two 
surfaces.  When  the  cutting-edges  are  shorter,  these  surfaces 
present  a  marked  general  convexity.  While  the  summit  of  the 
cusp  will  always  be  found  to  be  in  a  direct  line  with  the  long 
axis  of  the  tooth,  there  is  in  nearly  every  instance  a  difference  in 
the  length  of  the  cutting-edees,  and,  unlike  the  cutting-edares  of 
the  permanent  cuspid,  the  mesial  is  usually  the  longer.  The 
neck  of  the  tooth  is  much  constricted  and  the  root  straight  and 
conic. 

THE  UPPER  MOLARS. 

Upper  First  Molar. 


Fig.   169. 

Calcification  Begins,  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  Eighteenth  to  Twentieth  Month  after  Birth. 
Erupts,  Fourteenth  to  Fifteenth  Month  after  Birth. 
Decalcification  Begins,  Si.\th  to  Seventh  Year. 

Shedding  Process  Takes  Place,  about  the  Tenth  Year. 
Average  Length  of  Crown,  .20. 

Average  Length  of  Root,  .39. 

Average  Length  over  All,  .59. 


The  contour  and  lobate  construction  of  the  crown  of  this  tooth 


276  ANATOMY. 

is  peculiar  to  itself,  being  dissimilar  to  any  other  class  of  teeth 
in  the  mouth.  Calcification  takes  place  from  three  centers,  two 
for  the  buccal  and  one  for  the  lingual  half  of  the  crown.  The 
general  form  of  the  crown  may  best  be  studied  by  an  examination 
of  the  occlusal  surface. 

The  Occlusal  Surface  of  the  Crown. — The  outlines 
represented  are  those  of  an  irregular  quadrilateral,  of  which 
the  buccal  and  mesial  sides  are  the  longest.  The  angles  of 
the  quadrilateral  are  somewhat  variable,  the  mesiobuccal  being 
acute,  the  mesiolingual  obtuse,  while  the  two  distal  angles  are 
rounded  right  angles.  The  surface  is  surmounted  by  three 
cusps,  a  mesiobuccal,  a  distobuccal,  and  a  lingual.  These 
various  cusps  are  separated  from  one  another  by  three  develop- 
mental grooves — the  mesial,  the  distal,  and  the  buccal.  The 
marginal  ridges  are  sharp  and  well  defined,  this  being  particu- 
larly true  of  the  buccal  and  lingual,  which  resemble  cutting- 
edges.  The  mesio-marginal  ridge  begins  at  the  mesiobuccal 
angle,  and,  after  making  a  long  distal  curve,  ends  in  the  mesial 
incline  of  the  lingual  cusp.  The  center  of  the  surface  is  deeply 
and  Irregularly  concave,  producing  the  central  fossa,  and  de- 
scending from  the  various  ridges  and  cusps  surrounding  it  are 
numerous  supplemental  grooves  and  ridges.  The  various 
developmental  grooves  are  not  inclined  to  cross  the  marginal 
ridges  although  in  some  instances  one  or  two  may  be  found  to 
do  so. 

The  Buccal  Surface  of  the  Crown  (Fig.  169).— This  sur- 
face is  generally  smooth  and  convex,  with  an  excessively  devel- 
oped cervical  ridge,  which  is  particularly  prominent  at  its  mesial 
extremity.  The  buccal  groove  is  in  the  form  of  a  slight 
depression,  and  the  buccal  ridges,  common  to  all  molars,  are 
scarcely  to  be  observed.  The  mesial,  occlusal,  and  cervical  mar- 
gins are  distinctly  outlined,  while  the  distal  margin  is  obliterated 
by  the  gradual  passing  of  this  surface  into  the  distal  surface. 

The  Lingual  Surface  of  the  Crown. — This  surface  is 
circular  in  outline,  decidedly  convex  and  smooth,  and  is  seldom 
broken  by  grooves  and  ridges.  It  is  most  prominent  near  the 
center,  from  which  point  it  slopes  in  every  direction.  The 
cervical    ridge    is    not   so    pronounced    as    that   of    the   buccal 


THE  DECIDUOUS  TEETH    TN   DETAH.. 


surface,    but  there  is  a  sudden    rounding  of  tlie  surface   in   a 
cervical  direction  to  meet  the  Hngual  root. 

The  Mesial  Surface  of  the  Crown. — This  surface  is 
probably  more  extensive  than  any  of  the  others ;  it  is  inclined 
to  flatness,  with  a  slight  conic  convexity  over  its  occlusal 
third,  and  a  slight  concavity  near  the  cervix.  The  buccolingual 
measurement  of  the  surface  is  nearly  twice  as  great  as  that 
from  the  occlusal  margin  to  the  cervical  line.  It  is  much  more 
prominent  near  the  occlusal  margin,  so  that  a  V-shaped  space 
usually  exists  between  it  and  the  distal  surface  of  the  cuspid. 

The  Distal  Surface  of  the 
Crown.— The  extent  of  this 
surface  is  much  less  than  that 
of  the  mesial  ;  it  presents  a 
general  convexity,  and  is  sel- 
dom broken  by  grooves  or 
ridges,  although  occasionally 
the  distal  groove  crosses  its 
occlusal  margin.  Like  the  de- 
ciduous teeth  previously  de- 
scribed, the  neck  of  the  tooth 
is  marked  by  a  decided  and 
abrupt  constriction,  this  form 
appearing  to  arise  from  the 
heavy  enamel  folds  which  are 
present  near  the  cervical  line, 

rather  than  from  any  marked  constriction  in  the  base  of  the  roots 
themselves. 

The  roots  of  the  tooth  are  three  in  number — a  mesio- 
buccal,  a  distobuccal,  and  a  lingual ;  of  these,  the  latter  is 
usually  the  largest  and  longest.  The  two  buccal  roots  are 
much  flattened  from  mesial  to  distal,  while  the  lingual  is  com- 
pressed in  the  opposite  direction.  The  apical  ends  of  the  roots 
are  much  separated  from  one  another,  the  triangle  which  these 
points  form  being  almost  twice  the  size  of  the  triangle  formed 
by  the  base  of  the  roots.  The  apical  ends  are  usually  provided 
with  a  central  curve. 


9  ^ 


170. — Occlusal  Surfaces  of  the 
Deciduous  Molars. 


278 


Upper  Second  Molar. 


Fig.   171. 

Calcification  Begins,,  between  the  Fifth  and  Sixth  Fetal  Months. 

Calcification  Completed,  Twentieth  to  Twenty-second  Month  after  Birth. 
Erupted,  Eighteenth  to  Twenty-fourth  Month  after  Birth. 
Decalcification  Begins,  Seventh  to  Eighth  Year. 

Shedding  Process  Takes  Place,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .22. 

Average  Length  of  Root,  46. 

Average  Length  over  All,  .68. 


The  most  remarkable  feature  about  the  crown  of  this  tooth 
is  its  close  resemblance  to  the  crown  of  the  upper  perma- 
nent first  molar.  The  various  surfaces  are 
almost  identical,  the  developmental  process, 
and  consequently  the  cusp-formation,  is 
the  same,  the  marginal  and  other  ridges 
common  to  the  occlusal  surface  correspond, 
and  both  the  central  and  distal  fossae  are 
present,  together  with  the  various  develop- 
mental grooves.  A  description  of  the  crown 
will,  therefore,  be  unnecessary ;  suffice  it  to 
say  that  it  is  much  smaller  in  every  direction 
and  is  somewhatmore  constricted  at  the  neck. 
The  roots  are  the  same  in  name  and  number 
as  those  of  the  first  permanent  molar,  but 
they  are  more  widely  separated  at  their  apical  extremities.  In 
general  form  they  are  smaller  than  those  of  the  upper  first 
deciduous  molar. 


Fig.  172. — Upper  Sec- 
ond Molar,  Buccal 
Surface. 


THE  DECIDUOUS  TEETH   IN  DETAIL. 


THE  LOWER  DECIDUOUS  TEETH. 
A  description  in  detail  of  the  lower  incisors  and  cuspids 
would  practically  be  a  repetition  of  that  given  of  the  corre- 
sponding upper  teeth,  and  for  that  reason  will  be  passed  with 
a  limited  reference  to  each.  The  lower  molars  being  in  many 
respects  unlike  the  upper,  they  will  require  a  separate  de- 
scription. 

Lower  Central  Incisor  (Fig.  173). 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Sixteenth  to  Eighteenth  Month  after  Birth. 
Erupted,  Si.xth  to  Eighth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fourth  Year. 

Shedding  Process  Takes  Place,  about  the  Seventh  Year. 
Average  Length  of  Crown,  .19. 

Average  Length  of  Root,  .35. 

Average  Length  over  All,  .54. 

This  is  the  smallest  of  the  lower  teeth,  in  this 
respect  being  at  variance  to  the  upper  central, 
which  is  larger  than  the  lateral.  The  mesio- 
distal  diameter  of  the  crown  is  but  little  less 
than  that  from  the  cutting-edge  to  the  cervical 
line.  The  mesial  and  distal  angles  are  similar, 
both  being  pointed  and  square.  The  cervical 
ridge  is  quite  pronounced  and  the  neck  much  con- 
stricted. 

The  root  is  usually  straight  and  tapers  gradu- 
ally from  base  to  apex.  It  is  broader  on  the 
labial  than  on  the  lingual  side,  and  the  mesial 
and  distal  sides  are  but  little  flattened. 


Fig.   173.— 
Lower  Central 
Incisor,    Labial 
Surface. 


Lower  Lateral  Incisor  (Fig.  174). 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Twelfth  to  Fourteenth  Month  after  Birth. 
Erupted,  Seventh  to  Ninth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fifth  Year. 

Shedding  Process  Takes  Place,  about  the  Eighth  Year. 
Average  Length  of  Crown,  .19. 

Average  Length  of  Root,  .39. 

Average  Length  over  All,  .5S. 

This  tooth  is  larger  than  the  central  incisor,  and  closely  resem- 
bles the  upper  lateral  both  in  size  and  form.     The   crown   is 


ANATOMY. 


Fig.   174. — 
Lower  Lateral 
Incisor,    Labial 
Surface. 


more  rounded  in  its  nature  than  that  of  the  central,  forming  a 
greater  general  convexity  to  the  labial  surface,  and  less  con- 
cavity to  the  lingual.  The  mesial  angle  of  the 
crown  is  fairly  well  defined,  while  the  distal  is 
usually  much  rounded  by  a  long,  circular  sweep 
of  the  cutting-edge  to  meet  the  distal  surface. 
The  mesial  surface  of  the  crown  is  flattened  and 
somewhat  prominent  at  the  angle,  while  the  dis- 
tal surface  is  strongly  convex.  The  labial  grooves 
are  but  slightly  visible,  while  the  corresponding 
lingual  grooves  are  quite  pronounced.  The  neck 
of  the  tooth  is  even  more  marked  than  that 
of  the  lower  central  incisor.  The  root  is  long 
and  tapering,  slightly  flattened  from  mesial  to 
distal,  with  a  decided  longitudinal  groove  on 
both  the  mesial  and  distal  sides.  The  labial 
and  lingual  sides  are  rounded,  and  there  is  an  inclination  to 
crookedness,  which  is  usually  from  mesial  to  distal. 

Lower  Cuspid  (Fig.  175). 

Calcification  Begins,  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  about  Two  Years  after  Birth. 

Erupted,  Seventeenth  to  Eighteenth  Month  after  Birth. 
Decalcification  Begins,  about  the  Ninth  Year. 

Shedding  Process  Takes  Place,  about  the  Twelfth  Year. 
Average  Length  of  Crown,  .23. 

Average  Length  of  Root,  .45.  * 

Average  Length  over  All,  .68. 

The  principal  variations  between  this  tooth 
and  the  upper  cuspid  are  observed  in  the  dim- 
inished mesiodistal  measurement  of  the  crown, 
together  with  it  being  somewhat  less  angular 
in  outline.  The  ridges  and  grooves  common 
to  the  various  surfaces  are  not  so  marked  as 
those  of  the  upper  cuspid,  resulting  in  a 
smoothly  formed  crown  throughout.  The  root 
is  larger  in  proportion  to  the  size  of  the  crown 
than  that  of  its  upper  opponent,  thus  pro- 
ducing a  neck  much  less  constricted.  It  is 
usually  straight,  or  provided  with  a  slight  distal 


THE  DECIDUOUS  TEETH    IN  DETAIL.  281 

inclination  near  its  apical  extremity,  and  much  flattened  from 
mesial  to  distal,  these  two  sides  converging  to  the  lingual, 
forming  a  rounded  triangular  outline. 

Lower  First  Molar  (Fig.  176). 

Calcification  Begins  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  Eighteenth  to  Twentieth  Month  after  Birth. 
Erupted,  Fourteenth  to  Fifteenth  Month  after  Birth. 
Decalcification  Begins,  Si.\th  to  Seventh  Year. 

Shedding  Process  Begins,  about  the  Tenth  Year. 
Average  Length  of  Crown,  .24. 

Average  Length  of  Root,  .38. 

Average  Length  over  All,  .62. 

Upon  making  an  examination  of  the  occlusal  surface  of  this 
tooth  it  will  be  observed  that  the  crown  is  made  up   of  four 
irregularly  formed  lobes,   separated  from  one  another  by  four 
well-defined  grooves.     Each  lobe    is  pro- 
vided with  a  cusp,  more  or  less  prominently 
developed.     Between  the  various  cusps  are 
two  fossse — one  occupying  the  distal  two- 
thirds  of  the  surface  (the  distal  fossa)  and 
the  other  the  remaining  or  mesial  third  (the 
mesial  fossa).     The  outline  of  this  surface, 
which  represents  the  contour  of  the  crown  in 

general,  is  that  of  an  oblong  square,  with  its         Fig.    176. —  Lower 
,  ,  1111-  First  Molar,  Buccal 

angles  more  or  less  rounded,  and  havmg  a     surface. 

slight  variation  in  its  parallel  lines.     Each 

lobe  denotes  a  separate  center  of  calcification,  and    the    four 

grooves  the  lines  of  union  between  the  various  parts. 

The  mesiobuccal  lobe  is  somewhat  irregular  in  contour 
and  is  frequently  the  largest  of  the  four.  It  assists  in  forming 
the  mesiobuccal  angle  of  the  crown  and  the  greater  part  of  the 
mesial  fossa.  Descending  from  this  cusp  to  the  lingual  is  a 
pronounced  triangular  ridge,  which  is  made  continuous  by 
uniting  with  a  similar  ridge  from  the  corresponding  lingual 
cusp.  By  this  union  a  transverse  ridge  is  established,  separating 
the  mesial  from  the  distal  fossa.  The  central  boundary  of  this 
lobe  is  formed  by  the  mesial  groove,  which  arises  from  the  distal 
fossa,  passes  over  the  transverse  ridge  to  the  mesial  fossa,  from 


282  ANATOMY. 

which  it  continues  to  the  lingual,  and  by  the  buccal  groove, 
which  branches  off  from  the  mesial  somewhat  to  the  distal  of  the 
transverse  ridge,  passing  over  the  buccomarginal  ridge  to  the 
buccal  surface. 

The  Distobuccal  Lobe. — This  cusp  is  generally  smaller  than 
the  mesiobuccal,  and  is  more  pointed  and  more  regular  in  outline. 
It  assists  in  forming  the  distobuccal  angle  of  the  crown,  and 
by  its  central  incline  forms  about  one-third  of  the  distal  fossa.  Its 
boundaries  are  formed  by  the  buccal,  mesial,  and  distal  grooves, 
the  latter  beginning  in  the  distal  fossa,  and  passing  over  the 
distomarginal  ridge  to  the  distal  surface. 

The  Mesiolingual  Lobe. — In  the  recently  erupted  tooth 
the  summit  of  this  cusp  is  long  and  pointed,  and  frequently 
remains  the  most  pronounced  of  the  four.  It  is  triangular  in 
outline,  and,  as  above  referred  to,  furnishes  a  triangular  ridge, 
which,  by  uniting  with  a  like  ridge  from  the  mesiobuccal  cusp, 
forms  the  transverse  ridge.  By  its  central  incline  it  assists  in 
forming  the  mesial  fossa.  Its  boundaries  are  formed  by  the 
mesial  groove  and  the  lingual  groove,  the  latter  arising  near 
the  center  of  the  distal  fossa,  passing  to,  and  sometimes  cross- 
ing, the  linguomarginal  ridge. 

The  Distolingual  Lobe. — This  is  usually  the  smallest  of 
the  four.  It  is  inclined  to  be  rounded,  rather  than  angular,  and  in 
some  instances  is  poorly  developed.  It  assists  in  forming  the 
distolingual  angle  of  the  crown,  as  well  as  a  portion  of  the  distal 
fossa.  Its  central  boundaries  are  formed  by  the  lingual  and 
distal  grooves. 

The  marginal  ridges  of  the  surface  are  abruptly  but  irregularly 
formed,  asecndingand  descending  the  various  cusps  in  a  manner 
similar  to  those  previously  described. 

The  Buccal  Surface  of  the  Crown  (Fig.  176). — This  sur- 
face is  smooth  and  generally  convex,  with  a  mesiodistal  measure- 
ment about  twice  as  great  as  that  from  the  cervical  line  to  the 
occlusal  margin.  The  surface  is  most  prominent  over  its 
cervical  third,  forming  a  well-rounded  and  bold  cervical  ridge, 
a  feature  strongly  characteristic  of  this  tooth.  The  distal  center 
of  the  surface  is  broken  by  the  buccal  groove,  which  usually 
ends  near  the  center  in  a  shallow  depression  or  pit. 


THE  DECIDUOUS  TEETH   IN  DETAIL.  283 

The  Lingual  Surface  of  the  Crown. — This  surface  is 
much  less  extensive  than  the  buccal.  It  is  smooth  and  convex 
throughout,  and  is  broken  near  its  distal  center  by  the  lingual 
groove,  which  gradually  disappears  as  it  passes  rootward. 
The  cervical  ridge  is  not  so  prominent  as  that  of  the  buccal 
surface. 

The  Mesial  and  Distal  Surfaces  of  the  Crown. — These 
are  slightly  convex  in  every  direction,  the  former  passing, 
by  a  gradual  sweep,  into  the  lingual  surface,  destroying  the 
angularity  of  the  crown  at  this  point,  while  the  latter  passes 
more  abruptly,  forming  an  acute  angle.  These  surfaces  are  both 
prominent  near  the  occlusal  margin,  making  the  point  of  contact 
with  adjoining  teeth  near  that  surface.  The  bold  cervical  ridge 
of  the  buccal  surface  is  discontinued  or  greatly  diminished  upon 
these  surfaces,  both  of  which  are  inclined  to  pass  very  gradually 
Into  the  base  of  the  roots. 

The  roots  of  this  tooth  are  the  same  In  name,  position,  and 
number  as  those  of  the  lower  permanent  molars.  They  are 
much  flattened  from  mesial  to  distal,  the  center  of  their  flattened 
sides  being  further  compressed  by  a  deep  longitudinal  groove, 
which  extends  from  the  base  to  the  apex  of  each  root.  In  passing 
from  the  base  of  the  roots  to  their  apices  they  become  more 
widely  separated,  until  these  extremities  are  much  wider  apart, 
proportionately,  than  those  of  the  permanent  molars. 


Lower  Second  Molar  (Fig.  177). 

Calcification  Begins,  between  the  Fifth  and  Sixth  Fetal  Months. 

Calcification  Completed,  Twentieth  to  Twenty-second  Month  after  Birth. 
Erupted,  Eighteenth  to  Twenty-fourth  Month  after  Birth. 
Decalcification  Begins,  Seventh  to  Eighth  Year. 

Shedding  Process  Takes  Place,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .21. 

Average  Length  of  Root,  .44. 

Average  Length  over  All,  .65. 

The  anatomy  of  this  tooth  being  almost  identical  to  that  of 
the  lower  first  permanent  molar,  it  will  be  unnecessary  to 
enter  into  a  description  in  detail.  The  lobes,  and  consequently 
the  grooves,  are  the  same  in   position,  name,  and   number,  and 


28+  ANATOMY. 

a    similar    developmental    process    is    recorded.       The    tooth 
is  not  characterized  by  a  prominent  cervical  ridge,  such  as  is 
found  upon  the  lower  first  molar,  the  crown  passing  very  gradu- 
ally into  the  root-base  with  a  neck  moder- 
ately constricted. 

THE  PULP-CHAMBERS  AND  CANALS  OF 
THE  DECIDUOUS  TEETH. 
A  few  general  remarks  in  reference  to 
these  cavities,  in  connection  with  the  infor- 
mation to  be  derived  from  the  accompany- 
ing chart  and  its  annexed  description,  will 
sufficiently  instruct  the  reader,  without  the 
necessity  of  treating  each  tooth  individually. 
Second  molI^,^bu'ccal  The  pulp-chambers  and  canals,  like  those 
Surface.  ^^    ^.j-^g    permanent    organs,    assume    the 

form  of  the  external  contour  of  the  tooth, 
the  crown  of  the  tooth  being  provided  with  a  central  cavity, 
the  pulp-chamber,  partaking  of  outlines  closely  resembling  those 
of  the  crown,  while  the  root  is  traversed  by  the  pulp-canal,  like- 
wise conforming  to  the  shape  of  the  root.  One  very  important 
distinction  between  the  pulp-chambers  and  canals  of  the  de- 
ciduous teeth  and  those  of  the  permanent  organs  is  that  the 
former  are  proportionately  larger.  It  must  also  be  noted  that 
the  apical  foramina  in  these  teeth  are  so  transitory  in  their 
nature  that  there  remains  but  a  very  brief  period  during  which 
the  canals  may  be  said  to  be  fully  formed.  It  will  be  recalled 
that  in  a  very  short  time  after  the  roots  have  become  completely 
calcified,  decalcification  begins,  and  this  process  of  degeneracy, 
beginning  at  the  apical  extremities  of  the  roots,  very  early 
destroys  the  foramina,  which  have  in  a  measure  served  as  a  pro- 
tection to  the  surrounding  parts  during  operations  upon  the 
canals. 

With  the  canals  proportionately  larger  than  those  which  occupy 
the  roots  of  the  permanent  teeth,  the  foramina  during  their  very 
limited  existence  are  also  much  larger,  and  much  more  readily 
penetrated.  With  these  ever-changing  conditions  in  the  pulp- 
canals  of  the  deciduous  teeth,  it  is  of  importance  that  a  definite 


THE  DECIDUOUS  TEETH    IN  DETAIL. 


-85 


knowledge  of  what  takes  place  should  be  acquired,  and  it  is  for 
this  purpose  that  the  accompanying  chart  has  been  prepared 
(Fig.  178). 


Mo.  Old 
Twelve 
Mo.  Old 


Fig.  178. — Ch.^rt  showing  Calcification  and  Decalcific.a.tion  of  the  Deciduous  Teeth  and  xiiE 
Consequent  Length  of  the  Pulp-canal  at  Various  Periods. — {After  Peitxe.) 


CHAPTER  XII. 

DEVELOPMENT  OF  THE  TEETH— THE  DENTAL  GERMS,  ENAMEL 
ORGAN,  AND  DENTIN  ORGAN;  THE  DENTAL  FOLLICLE;  CALCI- 
FICATION AND  ERUPTION. 

DEVELOPMENT   OF  THE  TEETH. 


Neck  of  Cord 


Papilla 


Fig.   179. — Developing  Tooth-germ.     X  3°°- 


In  order  that  the  student  may  obtain  a  general  idea  of  the 
structural  changes  which  take  place  at  a  very  early  period,  and 
which  eventually  result  in  the  formation  of  the  teeth,  the  genesis 
of  the  subject  will  be  briefly  referred  to.  Preparation  for  the 
development  of  the  teeth  takes  place  as  early  as  the  middle  of 

2S6 


DEVELOPMENT  OF  THE  TEETH.  287 

the  second  fetal  month,  this  preparatory  alteration  in  the 
tissues  beginning  before  the  process  of  ossification  in  the  bony 
structures  which  finally  surround  and  give  support  to  the 
organs.  At  this  early  period  there  will  be  found  following 
the  line  of  the  future  alveolar  ridge  a  slight  heaping  up  of 
the  surface  epithelium,  while  immediately  beneath  this  pro- 
liferation of  cells  there  appears  a  dipping  in  of  the  deep 
epithelial  layer  in  the  direction  of  the  future  alveolar  walls. 
This  epithelial  reflection  is  known  as  the  epithelial  band  or 
tooth-band.  This  so-called  tooth-band  is  not,  as  might  be  sup- 
posed, a  special  inflection  for  each  tooth-germ,  but  is  continu- 
ous from  one  end  of  the  future  jaw  to  the  other.  It  must  be 
remembered  that  at  this  time  the  outline  of  the  jaw  has  not 
been  established,  and  the  tooth-band,  although  not  generally 
considered  as  essential  to  the  developmental  process,  is  princi- 
pally instrumental  in  directing  the  position  of  the  dental 
organs.  The  position  and  form  of  this  epithelial  band  may 
best  be  studied  in  vertical  transverse  section.  When  first 
making  its  appearance  it  is  somewhat  broad  and  shallow,  but 
as  it  passes  more  deeply  into  the  parts  it  partakes  of  the  out- 
line of  the  letter  V  with  its  open  end  directed  toward  the  sur- 
face. In  penetrating  the  subjacent  tissue,  the  free  extremity  of 
the  band  is  inclined  to  the  lingual,  its  external  surface  is  slightly 
convex,  and  its  internal  surface  correspondingly  concave.  His- 
tologically considered,  the  tooth-band  is  composed  of  elements 
similar  to  those  which  serve  to  make  up  the  epithelial  layer 
of  the  oral  mucous  membrane.  After  the  tooth-band  has 
assumed  certain  proportions,  there  appears  on  its  inner  or  con- 
cave surface  a  thin  niembranous  plate,  which  is  likewise  a 
continuous  structure,  extending  the  full  length  of  the  epithelial 
band. 

This  lamina  does  not  spring  from  the  free  margin  of  the  tooth- 
band,  but  is  given  off  at  a  point  about  midway  between  this 
border  and  the  base  of  the  band.  The  structure  of  this  secondary 
band  is  so  similar  to  that  of  the  primary  one  that  it  should  be 
considered  as  an  inflexion  from  it  rather  than  a  new  structure. 
We  find  then  between  the  seventh  and  eighth  week,  the  maxillary 
regions  giving  place  to  two  bow-shaped  bands  (one  for  each 


288       '  ANATOMY. 

jaw),  each  of  which  is  preparing  to  throw  out  from  its  secondary 
lamina  ten  Httle  buds,  which  soon  develop  into  the  germs  for  the 
twenty  deciduous  teeth.  When  these  buds  make  their  appear- 
ance they  are  simple,  rounded  bodies,  placed  somewhat  closely 
together,  but  they  do  not  long  retain  this  simple  form.  The 
first  change  which  takes  place  is  one  in  which  they  appear  to 
lengthen  out  into  slender  cords,  the  extremities  of  which  soon 
begin  to  extend  laterally,  and  the  primitive  enamel  organ  is 
formed.     Accompanying  this  lateral  extension  of  the  periphery, 


Ef  itl  ehum  of  Lower  Ja 


Tlellate  Retic- 


j      L  11  imel  Organ 
Dentin  Papilla 


De\  eloping  Tooth    ei  \      1  \\  llfth  \\  eeiv      >   40. 


a  bell-shaped  outline  is  assumed,  which  phenomenon  is  rapidly 
increased  by  a  specialization  of  the  surrounding  cellular  tissue 
forcing  into  the  concavity. 

This  bell-shaped  proliferation  of  cells,  given  off  directly  from 
the  tooth-band,  to  which  it  continues  for  a  time  attached, 
together  with  the  specialized  connective  tissue  crowding  into  its 
concavity,  constitute  the  tooth-germs,  the  former  being  the  enamel 
organ,  and  the  latter  the  dentin  organ.  It  will,  therefore,  be  seen 
that  the  enamel  is  dependent  upon  the  oral  epithelium  for  its 
development  (ecdermic),  while  the  dentin   springs  from  an  en- 


DEVELOrMENT  OF  THE  TEETH.  289 

tirely  different  source — the  connective  tissue  of  the  jaw  (ender- 
mic).  The  enamel  organ  rapidly  undergoes  a  cellular  transfor- 
mation :  its  concavity  is  increased,  and  the  bell-shaped  outline 
more  strongly  defined.  Accompanying  this  change  in  form  it 
gradually  recedes  from  the  surface,  and  its  connection  with  the 
tooth-band  becomes  less  secure.  The  connective-tissue  cells, 
which  have  been  rapidly  filling  in  the  concavity  of  the  enamel 
organ,  are  also  preparing  to  take  upon  themselves  a  special 
function,  that  of  the  formation  of  the  dentin.  Up  to  this  period 
(tenth  week)  the  enamel  and  dentin  germs  are  not  definitely 
separated  from  the  surrounding  cellular  structure,  but  now  a 
gradual  transformation  takes  place,  whereby  the  tooth-germs 
become  enveloped  in  a  sac-like  covering — the  dental  follicle. 

Enamel  Organ  (Figs.  179,  180,  and  181). — This  portion  of 
the  tooth-germ,  as  previously  stated,  is  derived  from  the  concave 
or  lingual  surface  of  the  tooth-band,  which  in  turn  is  derived  from 
the  surface  epithelium.  From  the  free  extremity  of  its  slender 
cord-like  attachment  it  spreads  out  and  forms  a  hood-like  cover- 
ing to  the  dentin  germ.  The  surface  of  the  organ  contiguous  to 
the  dentin  germ,  or  dentin  papilla,  as  it  is  frequendy  called,  is 
concave  in  the  direction  of  the  oral  surface,  being  thickest  over 
the  center  of  its  concavity,  thinning  down  as  its  periphery  is 
approached.  Externally,  the  enamel  organ  is  covered  by  an 
epithelial  layer,  which  is  reflected  upon  its  inner  surface  or  that 
in  contact  with  the  dentin  papilla.  These  two  layers  are  named 
according  to  their  location,  the  external  and  internal  epithelium 
of  the  enamel  organ.  Placed  between  these  two  layers,  and 
constituting  the  bulk  of  the  organ,  are  numerous  stellate  bodies 
which  penetrate  a  layer  of  rounded  cells,  the  stratum  intervie- 
diiim,  and  finally  reach  the  internal  epithelial  layer  known  as  the 
enamel  cells  or  ameloblasts.  It  is  from  this  internal  layer  of  epi- 
thelial cells  that  the  enamel  is  calcified,  and  they  are,  therefore, 
the  essential  cells  of  the  enamel  organ.  In  the  fully  developed, 
enamel  organ,  there  are  to  be' found,  therefore,  four  distinct  lay- 
ers of  cells,  the  external  epithelium,  stellate  reticulum,  stratum 
intermedium,  and  internal  epithelium,  ox  ameloblasts .  As  its  name 
implies,  the  function  of  the  enamel  organ  is  principally  that  of 
enamel  calcification,  but  in  the  opinion  of  many  writers  its  pri- 
19 


mary  activity  is  that  of  molding  the  tooth-form  as  represented 
by  the  dentin  papilla,  and  it  is  not  until  this  latter  organ  has 
assumed  the  form  and  extent  of  the  future  tooth-crown  that 
dentin  calcification  begins. 

The  life  of  the  enamel  organ  may  properly  be  considered  as 
beginning  when  the  bulbous  extremity  of  the  specialized  cells 
given  off  from  the  lingual  face  of  the  tooth-band  become  invag- 
inated,  and  from  this  by  a  rapid  proliferation  of  its  cells  it  passes 


Fig.  i8i. — Developing  Tooth-germs,  Enamel  Organ,  and  Dentin  Papilla. 


on  by  successive  stages  assuming  the  various  forms  common  to 
it.  This  proliferation  and  differentiation  ot  cells  continues  up 
to  the  time  of  beginning  of  calcification,  but  with  the  advent  of 
this  phenomenon  certain  parts  of  the  organ  begin  to  degenerate. 
This  degeneration  may  or  may  nc)t  be  classed  as  an  atrophy  of 
the  cells  interested,  but  the  fact  that  a  new  tissue  is  generating, 
and  gradually  occupying  the  space  previously  taken  up  by  the 
formative  cells,  calls  forth  a  demand  for  the  removal  of  the  latter 
by  the  former.  The  cells  which  first  undergo  this  change  are  those 


DEVELOPMENT  OF  THE  TEETH.  291 

of  the  internal  epithelium  and  stratum  intermedium,  the  individ- 
uality of  these  two  layers  evidently  being  kept  up  by  migratory 
cells  from  the  stellate  reticulum.  It  is  argued  by  some  writers 
that  the  external  epithelium  begins  to  atrophy  at  this  period ; 
by  others  this  change  is  not  recorded  until  the  enamel  cuticle 
has  been  deposited  to  effectually  seal  the  young  tissue  and  pro- 
tect it  until  well  desiccated.  While  there  appears  to  be  a  decided 
disposition  upon  the  part  of  this  outer  layer  of  cells  to  change, 


Fig.  1S2. — Developing  Tooth-germs.     Longitudinal  Sections  from  Buccal  to 
Lingual. 


they  do  not  disappear,  and  the  alteration  is  not  one  which  affects 
the  shape  of  the  cells,  for  they  remain  flattened  or  prismatic 
with  their  long  axis  placed  parallel  with  the  anlage  of  the  crown. 
The  stellate  cells  making  up  the  bulk  of  the  organ  are,  in  com- 
mon with  those  which  inclose  them,  continually  undergoing  a 
degenerative  change,  at  least  this  is  true  of  those  cells  closely 
associated  with  the  stratum  intermedium,  for   in   this   location 


they  rapidly  proliferate,  shed  their  many  processes,  and  grad- 
ually take  on  the  characteristics  common  to  this  layer  of  which 
they  eventually  become  a  part.  A  careful  examination  of  no 
less  than  one  hundred  enamel  organs  in  all  stages  of  develop- 
ment, and  by  section  cut  transversely,  obliquely,  longitudinally, 
etc.,  fully  justifies  the  statement  that  the  real  life  of  the  enamel 
organ  begins  as  previously  stated,  and  continues  until  the 
structural  arrangement  of  the  enamel  is  completed. 


Fig.   183. — Developing  Tooth-gerjis.     Longitudinal  Section  from  Mesial 
TO  Distal. 


The  question  of  form  in  the  enamel  organ — that  is,  its  exter- 
nal epithelium — is  one  which  may  be  advantageously  used  in 
the  consideration  of  the  life  and  function  of  its  different  cell 
layers.  It  has  been  said  that  the  apparently  extravagant  area 
taken  up  by  the  enamel  organ  subserves  the  purpose  of  reserv- 
ing space  for  the  growing  tooth-crown,  but  there  are  many 
reasons  why  this  theory  cannot  be  accepted.     In  the  first  place. 


DEVELOPMENT  OF  THE  TEETH.  293 

the  extent  of  the  organ  or  the  space  existing  between  the  den- 
tin papilla  and  the  outer  enamel  epithelium  does  not  in  very 
many  instances  correspond  to  the  bulk  of  enamel  when  this 
tissue  is  completed  at  a  given  point.  In  the  developed  tooth 
we  find  the  enamel  thickest  over  the  cutting-edges  of  the  ante- 
rior teeth  and  about  the  summits  of  the  cusps  of  the  cuspidate 
teeth,  while  these  same  parts  are  represented  during  the  cell- 
ular stage  of  development  by  the  external  layer  of  cells  closely 
associated  with  the  surface  of  the  papilla. 

Again,  in  very  many  instances  the  outline  of  the  tooth  is 
definitely  represented  by  the  cells  making  up  the  dentin  papilla 
(Figs.  I  Si  and  182),  but  the  surrounding  epithelial  cells  are 
characterized  by  an  unbroken  semicircular  margin  describing 
the  extent  and  form  of  the  enamel  organ.  Exception  may  be 
taken  to  this  hypothesis  from  the  standpoint  of  generative 
changes,  and  these  in  a  great  measure  have  much  to  do  with  the 
relative  outlines  assumed  by  the  two  organs,  but  by  studying 
very  many  sections  representing  nearly  every  stage  of  the 
process,  and  all  of  them  in  a  measure  showing  the  same  char- 
acteristics, nothing  but  a  definite  opinion  can  result. 

Of  the  many  changes  in  general  form  which  the  enamel  organ 
undergoes,  none  are  so  pronounced  and  positive  in  character  as 
those  described  by  the  inner  tunic,  and  first  recorded  when  the 
bulbous  end  of  the  specialized  cells  becomes  invaginated  by  the 
mesodermic  connective-tissue  cells  forcing  themselves  into  it. 
This  is  an  alteration  which  is  gradual  and  continuous  up  to  the 
time  of  beginning  of  calcification,  and  while  the  cells  forming 
the  dentin  papilla  are  generally  accorded  the  power  of  "push- 
ing" or  "forcing"  their  way  into  those  derived  from  the  epiblast, 
the  latter  have  always  been  recognized  as  having  a  controlling 
influence  over  the  former.  In  this  connection  a  reasonable 
doubt  presents  itself  covering  the  theory  so  long  accepted  that 
the  early  function  of  the  enamel  organ  is  one  which  in  a  meas- 
ure superintends  the  contouring  of  the  tooth-crown  as  first  rep- 
resented in  the  dentin  papilla.  When  the  character  of  the 
two  embryonal  tissues  making  up  the  two  germs  is  compared, 
we  find  the  dentin  germ  possessing  all  the  characteristics  favor- 
able to  a  rapid   proliferation  of  its  cells  resulting  in  a  highly 


vascular,  compact  tissue.  On  the  other  hand,  the  bulk  of  the 
enamel  organ  is  a  gelatinous-like  mass,  one  that  would  readily 
succumb  to  the  pressure  exerted  by  those  active  connective- 
tissue  cells  within  its  borders.  When  thus  considered,  the  evi- 
dence is  almost  sufficiently  convincing  to  reverse  the  generally 
accepted  theory,  placing  the  general  form  of  the  enamel  organ 
as  subservient  to  the  dentin  papilla. 


Fig.   184. — Developing  Tooth-germs  in  Transverse  Section. 
A,  Stellate  reticulum.     B,  Papilla.     C,  Cartilage  cells. 


Figures  181,  182,  and  183  illustrate  some  of  the  variations 
common  to  the  general  form  of  the  enamel  organ,  and  afford  a 
good  idea  of  the  relationship  existing  between  the  enamel  organ 
and  the  dentin  papilla,  in  teeth  both  of  the  simple  and  complex 
class.  These  were  taken  from  sections  which  represent  a  period 
just  prior  to  the  generation  of  the  ameloblasts  and  odontoblasts, 
at  which  time  the  external  and  internal  epithelial  layers  of  the 
enamel  orean  most  closelv  resemble  one  another  in  greneral  out- 


DEVELOPMENT  OF  THE  TEETH. 


295 


line.  It  is  from  this  aspect  and  from  sections  cut  longitudinally 
that  most  of  the  information  given  by  the  older  writers  has  been 
derived.  Few  attempts  have  been  made  to  show  this  organ  in 
section  transverse  to  the  long  axis  of  the  tooth  anlage.  In 
figure  184  the  germs  of  two  teeth  are  shown  by  a  section  made 
in  this  direction.  One  striking  feature  here  illustrated  is  the 
relationship  existing'between  the  inner  and  the  outer  tunic  of 


a 


'<-..  fS*- 


'4 


■**"*^.  *'^" 


^'^'1%^«7 


Fig.   1S5. 
A,  Inner  tunic  of  enamel  organ,     B,  Cells  of  dentin  papilla. 


■  3. 


the  enamel  organ,  and  attention  is  called  to  the  apparent  coales- 
cence of  these  two  layers  at  those  points  which  represent  the 
mesial  and  distal  surfaces  of  the  developing  crowns.  This  con- 
dition is  apparently  brought  about  by  the  cartilage  cells  forcing 
the  peripheral  cells  of  the  enamel  organ  into  direct  contact  with 
the  inner  tunic,  completely  obliterating  the  stellate  reticulum  in 
these  localities.  As  a  result  of  this  lateral  pressure  the  outer 
epithelial    cells    representing    the    labial    and    lingual    surfaces 


296  ANATOMY. 

have  become  widely  separated,  but  with  no  perceptible  alter- 
ation in  the  character  of  the  cells  composing  the  stellate 
reticulum.  The  relationship  existing  between  the  tooth-germs 
and  the  surrounding  parts  is  one  that  will  continue  throughout 
the  generation  of  the  organs,  and  makes  questionable  the  theory 
that  the  stellate  reticulum  performs  the  function  of  reinforcing 
or  providing  the  ameloblasts  with  nutrient  or  calcific  material. 


:ff^ 


Fig.  1S6. 
A,  Cells  of  dentin  papilla.      B,  Elongated  cells  of  inner  tunic. 

If  these  same  germs  were  examined,  as  they  usually  are,  in 
longitudinal  section  (see  Figs.  181,  182,  and  183),  the  investi- 
gator would  at  once  arrive  at  the  conclusion  that  there  was  an 
equal  distribution  of  the  stellate  cells  about  all  sides  of  the 
dentin  papilla.  In  their  very  early  life  they  apparently  establish 
an  equal  bulk  about  all  sides  of  the  dentin  germ,  but  with  the 
preparation  for  the  growth  of  the  alveolar  walls  they  may  assume 
the  proportions  shown  in  figure  184.     At  a  period  correspond- 


DEVELOPMENT  OF  THE  TEETH.  297 

ing  to  the  complete  envelopment  of  the  dental  germs  by  the 
dental  follicle  the  development  of  the  buccal  and  lingual  walls 
is  well  under  way,  but  as  yet  no  provision  has  been  made  for 
the  septa  between  the  teeth,  and  it  is  undoubtedly  to  the 
approach  of  this  latter  phenomenon  that  a  definite  lateral  pres- 
sure is  brought  to  bear  upon  the  approaching  walls  of  the  fol- 
licles. 

Now  let  us  pass  to  a  consideration  of  some  of  the  charac- 
teristics of  the  various  cell  layers  composing  the  enamel  organ. 
These  are  designated  according  to  their  location,  so  far  as 
three  of  the  four  layers  are  concerned;  in  fact,  the  remaining 
cells,  or  those  which  receive  their  name  from  their  form,  can 
scarcely  be  classified  as  a  distinct  layer,  these  stellate  cells  not 
being  of  uniform  thickness  in  all  parts  of  the  organ.  The  first 
layer  of  cells,  or  those  making  up  the  inner  tunic,  will  be  traced 
from  their  primary  spherical  condition  to  their  final  generation 
into  ameloblasts.  Figure  185  shows  the  character  of  these  cells 
at  a  very  early  period,  corresponding  to  the  sixteenth  week  in 
the  human  fetus.  They  are  for  the  most  part  spherical  or  slightly 
oblong  multinucleated  cells,  and  are  more  or  less  closely  asso- 
ciated. They  partake  very  much  of  the  nature  of  the  connec- 
tive-tissue cells  surrounding  them,  being  differentiated  from 
these  principally  by  a  transparent  zone  not  unlike  the  special- 
ized matrix  immediately  surrounding  cartilage  cells.  About 
the  first  change  recorded  in  these  cells  (see  Fig.  1S6)  is  one  in 
which  they  become  markedly  elongated  or  cylindrical,  but  dur- 
ing this  process  of  differentiation  some  of  the  cells  apparently 
recede,  while  others  advance  in  the  direction  of  the  papilla,  lin- 
ing up  in  a  single  layer  to  become  the  early  enamel  cells,  the 
cells  which  have  been  thus  forced  to  the  rear  subsequently 
developing  into  ameloblasts  as  the  older  cells  atrophy.  At  this 
period  the  stratum  intermedium  also  asserts  itself  in  the  form  of 
a  distinct  layer  of  rounded  cells,  to  be  described  later  on.  When 
first  observed,  these  cylindrical  cells  are  devoid  of  processes,  but 
are  provided  with  rounded  extremities,  with  little  or  no  variation 
between  the  end  directed  toward  the  papilla  and  that  looking  in 
the  opposite  direction.  This  form  is  one  which  persists  in  all 
of  the  cells  included  in  this  layer  until  a  definite  body  of  cells  is 


298 


ANATOMY. 


formed  contiguous  to  the  dentin  papilla,  these  latter  cells  be- 
coming more  markedly  elongated  and  further  differentiated  by 
the  addition  of  processes,  while  the  remaining  cells,  or  those 
nearest  the  stratum  intermedium,  continue  for  a  time  unchanged. 
The  next  alteration  in  the  character  of  the  inner  tunic  is  one 
well  illustrated  in  figure  186,  in  which  the  body  of  the  generat- 


A,  Stellate  reticulum.     B,  Stratum  intermedium.     C,  Ameloblasts.     D,  Forming  enamel. 
E,  Calcified  dentin.     F,  Odontoblasts. 


ing  ameloblastic  cells  rapidly  recede  from  the  surface  of  the 
papilla,  while  the  elongating  processes  reach  out  to  this  latter 
point,  all  of  this  occurring  before  the  appearance  of  the  odonto- 
blasts. Soon  after  this  latter  change  in  the  cells  of  this  layer 
the  odontoblasts  are  developed  and  form  the  periphery  of  the 
dentinal  tissue.     All   of  these  changes  are   of  course  first  re- 


DEVELOPMENT  OF  THE  TEETH.  299 

corded  about  the  free  extremity  of  the  tooth-crown,  becoming 
less  noticeable  as  the  union  of  the  outer  and  inner  tunics  is 
approached. 

A  study  into  the  special  characteristics  of  the  fully  developed 
ameloblasts  shows  that  these  active  cells  are  the  result  of  a 
gradual  change  in  the  character  of  the  columnar  epithelia  com- 


A,  Generating  ameloblasts. 


B,  Rounded  cells  of  stratum  intermedium.     C,  Stellate 
reticulum. 


mon  to  both  the  external  and  internal  epithelial  layers  in  the 
primitive  enamel  organ. 

Next  in  importance  to  the  internal  epithelial  layer  are  those 
closely  associated  cells  making  up  the  stratum  intermedium  (Fig. 
188).  Primarily  oval  or  spheroidal  in  form,  we  find  these  cells 
gradually  assuming  a  columnar  outline  and  occupying  a  position 
parallel  to  the  long  axis  of  the  crown.  It  may  be  said  that  the 
general  character  of  these  cells  is  intermediate  between  those 


destined  to  become  the  proper  enamel  cells  and  those  stellate 
cells  making  up  the  bulk  of  the  organ.  There  appears  to  be 
much  confusion,  at  least  considerable  doubt,  in  regard  to  the 
office  of  the  cells  of  the  stratum  intermedium.  It  is  most  likely 
that  these  cells  are  not  only  intermediate  in  character,  but  are 
also  intermediate  in  function  to  those  cells  upon  either  side  of 
them,  recruiting  the  ameloblasts  as  they  fall,  while  in  turn  they 
themselves  are  supplied  with  nutriment  from  the  enamel  pulp 
or  stellate  reticulum.  No  stronger  proof  that  these  cells  are 
secondary  in  importance  to  the  ameloblasts  need  be  mentioned 
than  reference  to  the  fact  that  they  are  always  more  generously 
supplied  to  those  parts  about  to  undergo  calcification.  Nor  is 
their  increase  in  numbers  the  only  reason  for  believing  that  they 
are  thus  employed,  for  at  the  same  time  those  cells  most  closely 
associated  with  the  developing  ameloblasts  take  upon  them- 
selves a  decided  change  in  outline.  This  alteration  may  be 
brought  about  by  the  conditions  which  influence  the  shapes  of 
all  cells,  i.  e.,  by  the  pressure  of  surrounding  cells  or  by  their 
preparation  for  functional  activity,  or  both.  Some  of  the  older 
writers  speak  of  the  cells  of  this  layer  as  being  branched,  and 
in  this  way  closely  resembling  those  of  the  stellate  reticulum. 
By  strong  amplification  it  is  somewhat  difficult  to  distinguish 
between  the  two  layers,  but  most  certainly  if  there  are  branched 
cells  present  they  are  confined  to  the  intermediate  zone,  and 
should  properly  be  classed  with  those  of  the  stellate  reticulum. 
The  cells  of  this  layer  do  not  long  remain  columnar  with  a 
general  direction  at  right  angles  to  the  forming  ameloblasts,  but 
as  these  latter  cells  appear  they  become  spheroidal  and  ex- 
tremely closely  associated  in  the  deeper  portion  of  the  layer  ;  in 
fact,  cells  corresponding  to  these  in  general  appearance  may  be 
found  in  connection  with  the  fully  developed  ameloblasts,  being 
observed  to  best  advantage  by  the  aid  of  a  high  power  objective 
and  a  full  flood  of  light  from  a  powerful  sub-stage  condenser. 
The  cells  thus  found  appear  to  be  distributed  at  regular  inter- 
vals about  the  ameloblastic  layer,  and  are  so  closely  allied  to  the 
cells  of  the  stratum  intermedium  that  they  may  be  considered  as 
migratory  cells  from  this  layer.  In  the  earlier  stages  there  ap- 
pears to  be  no  definite  line  of  demarcation  between  the  cells  of 


DEVELOPMENT  OF  THE  TEETH.  301 

the  inner  tunic  and  those  composing  the  stratum  intermedium, 
but  soon  after  the  establishment  of  the  ameloblasts  the  two  lay- 
ers are  strongly  differentiated  by  the  interposition  of  a  highly 
transparent  membrane  covering  the  outer  extremities  of  the 
ameloblasts.  After  they  are  thus  definitely  separated  from  the 
enamel-forming  cells,  a  most  radical  change  takes  place  in  their 
character ;  they  become  markedly  elongated,  and  by  anasto- 
mosing form  a  series  of  continuous  chain-like  belts  about  the 
ameloblastic  layer,  the  number  and  further  character  of  which  are 
dependent  upon  the  extent  to  which  the  ameloblasts  have  per- 
formed their  function.  If  at  any  time  there  is  a  similarity 
between  the  cells  of  the  stratum  intermedium  and  the  stellate 
reticulum  it  is  at  this  period,  because  the  former  cells  begin  to 
lose  their  individuality,  although  under  low  power  they  still 
appear  as  a  distinct  layer  (Fig.  187). 

There  is  probably  no  body  of  cells  directly  interested  in  the 
development  of  the  tooth  tissues  so  widely  discussed  as  those 
making  up  the  so-called  stellate  reticulum,  and  while  the  chief 
basis  for  argument  has  been  with  reference  to  their  function,  the 
general  character  and  form  of  the  cells  have  received  but  little 
consideration.  Ever  since  the  first  description  of  this  portion  of 
the  enamel  organ  the  cells  therein  have  been  characterized  as 
"star-shaped,"  and  while  this  stellate  form  is  the  most  common, 
it  is  by  no  means  a  universal  condition.  The  form  of  the  cells 
in  common  with  the  other  cells  composing  the  organ  appears  to 
be  much  influenced  by  the  position  which  they  occupy,  and  by 
the  age  of  the  organ,  those  cells  in  the  region  of  the  inner  tunic 
partaking  of  the  globular  form  characteristic  of  this  layer  (see 
C,  Fig.  188),  while  those  closely  associated  with  the  outer  tunic 
are  inclined  to  be  columnar  or  somewhat  elongated.  While  the 
bodies  of  the  cells  in  these  respective  locations  are  more  or  less 
influenced  by  their  environments,  they  still  retain  to  a  certain 
extent  the  stellate  feature  by  their  many  processes.  But  it  is  in 
the  center  of  this  myxomatous  epithelial  product  that  the  most 
perfect  stellate  cells  are  located.  We  find,  therefore,  where  this 
part  of  the  organ  is  of  the  greatest  width,  that  the  true  stellate 
cells  are  the  most  numerous,  while  at  the  summit  of  the  crown 
and  at  the  base  of  the  organ,  at  both  of  which  points  the  outer 


302  ANATOMY. 

and  inner  epithelial  layers  are  closely  associated,  the  star-shaped 
cells  are  almost  wanting.  In  the  study  of  this  layer  very  much 
depends  upon  the  thickness  of  the  section,  only  the  thinnest 
possible  sections  affording  an  opportunity  for  a  correct  concep- 
tion. This  is,  of  course,  true  of  all  parts  of  the  organ,  but  the 
peculiar  character  of  the  stellate  reticulum  makes  it  especially 
necessary  that  great  care  be  bestowed  upon  the  preparation  of 
the  section.  In  transverse  section  the  cells  present  no  charac- 
teristic differences  from  those  shown  when  the  section  is  made 
longitudinally.  One  very  pronounced  feature  about  the  cells  of 
the  stellate  reticulum  is  the  granular  appearance  of  their  proto- 
plasm, resembling  very  closely  the  flattened  squamous  cells  from 
the  epithelium  of  the  mouth,  and  it  is  no  doubt  this  special  fea- 
ture which  furnishes  the  ground  for  the  opinion  of  many  writers 
that  it  is  a  peculiarly  modified  epithelium.  One  peculiarity  in 
connection  with  this  dssue  which  is  contrary  to  the  generally 
accepted  character  of  epithelial  cells  is  the  abundance  of  inter- 
cellular cement  substance  ;  but  when  the  many  minute  spines  or 
processes  are  considered  as  a  part  of  the  individual  cell,  the 
proportionate  quantity  of  cellular  and  intercellular  substance  is 
somewhat  decreased.  The  connecting  processes  are  quite  sim- 
ilar to  those  described  by  Stohr  as  connecting  bridges  of  proto- 
plasm, while  the  cells  themselves  may  be  otherwise  described  as 
prickle-cells.  The  change  in  the  form  of  the  cells  of  this  layer 
is  not  due  to  the  presence  of  neighboring  cells,  as  is  the  case 
with  most  epithelial  cells,  but,  being  soft  and  extremely  plastic, 
it  is  more  than  likely  that  their  form  is  strongly  influenced  by 
the  tension  of  their  connecdng  filaments.  One  of  the  most 
marked  alterations  in  the  general  character  of  this  part  of  the 
enamel  organ  is  that  which  takes  place  at  a  time  corresponding 
to  the  beginning  of  amelification,  and  is  no  doubt  attributable 
to  this  phenomena.  The  cells  which  up  to  this  period  have 
remained  widely  separated  now  become  more  closely  associated, 
not  so  much  by  a  change  of  position  as  by  what  appears  to  be 
an  increase  in  the  size  of  the  cell  body  with  a  corresponding 
decrease  in  the  length  of  the  anastomosing  processes. 

It  is  a  fact  admitted  by  most  histologists  that  the  peculiar  star- 
like nature  of  the  cells  of  the  stellate  reticulum  is  one  princi- 


DEVELOPMENT  OF  THE  TEETH.  303 

pally  brought  about  by  postmortem  changes,  and  that  in  reality 
they  are  polygonal  cells  filling  up  a  greater  part  of  the  tissue 
with  but  little  intercellular  substance.  That  some  shrinkage  and 
distortion  does  take  place  may  be  proven  by  the  examination  of  a 
section  which  has  accidentally  or  otherwise  become  for  a  moment 
dry  during  its  preparation,  in  which  case  litde  can  be  seen  but 
the  connecting  processes,  and  even  these  are  much  shrunken. 
All  the  cells  contained  within  the  organ  are  more  or  less  affected 
by  this  procedure,  but  none  of  them  exhibit  such  a  marked  change 
in  the  outline  as  those  of  the  stellate  reticulum. 

The  layer  of  cells  which  is  usually  considered  of  least  impor- 
tance is  that  which  makes  up  the  outer  tunic.  In  the  young 
enamel  organ  the  cells  partake  very  much  of  the  nature  of  those 
forming  the  inner  tunic,  but  the  older  the  organ  becomes,  the 
more  dissimilar  are  the  two  layers.  Primarily  this  layer  is  con- 
structed of  a  single  row  of  elongated  cells,  placed  with  remark- 
able regularity,  upon  the  inner  side  of  which  are  a  number  of 
similarly  formed  cells  variously  disposed,  but  with  a  common 
direcdon  at  right  angles  to  those  previously  referred  to.  Like 
the  internal  epithelial  layer,  the  cells  of  the  outer  tunic  partake  , 
more  or  less  of  the  nature  of  the  stellate  cells  in  passing  from 
the  single  row  of  well-defined  cells  toward  the  stellate  reticulum. 
While  in  the  beginning  the  external  epithelial  layer  is  strongly 
differentiated  from  the  surrounding  cells,  this  is  of  but  short 
duration.  The  atrophy  of  this  layer  begins  with  the  appearance 
of  the  fully  developed  ameloblasts,  by  which  the  regular  arrange- 
ment of  the  cells  is  greatly  disturbed  by  an  apparent  breaking 
up  of  the  entire  layer.  Many  reputable  writers  claim  that  the 
external  epithelial  layer  is  of  little  or  no  interest,  save,  as  Tomes 
puts  it,  "as  a  matter  of  controversy."  This  admission  upon  the 
part  of  so  eminent  an  authority  practically  opens  up  a  new  field 
for  research,  especially  so  when  we  consider  that  various  other 
writers  (Waldeyer,  Kolliger,  and  Magitot)  have  expressed  con- 
flicdng  opinions  in  regard  to  it.  After  carefully  following  the 
changes  which  occur  in  this  layer  from  its  earliest  inception  up 
to  an  advanced  stage  of  calcification,  it  would  appear  that  while 
marked  changes  occur  in  the  character  of  the  individual  cells  as 
well  as  in  the  general  appearance  of  the  layer,  it  is  nevertheless 


304  AJNAlUMi. 

persistive,  and  in  some  way  is  essential  to  the  process  of  amelifi- 
cation,  even  to  a  more  marked  degree  than  are  the  cells  of  the 
stellate  reticulum.  One  important  reason  for  this  belief  is  based 
upon  the  fact  that  in  the  cuspidate  teeth  there  appears  at  a  time 
corresponding  to  the  beginning  of  amelification,  a  decided  dis- 
position in  the  cells  of  this  layer  to  dip  down  and  completely 
divide  the  stellate  reticulum  between  the  forming  cusps.  That 
this  alteration  is  one  instrumental  or  essential  to  the  calcifying 
process  receives  additional  proof  by  referring  to  figure    i8i, 


Fig.   189.— Section  through  thi,  I  i..,.i;  ,.,    mi.  Mouth  of  Human  Embryo. 
Twelfth  Week.     ,  ,  30. 

which  shows  the  fully  developed  enamel  organ  with  the  excep- 
tion of  the  actual  appearance  of  the  ameloblasts,  and  the  lack 
of  any  attempt  upon  the  part  of  the  external  epithelial  layer  to 
penetrate  between  the  cusps. 

Dentin  Organ  (Fig.  iSo). — This  part  of  the  tooth-germ, 
formed  from  the  connective  tissue  of  the  primitive  jaw, 
occupies  the  concavity  of  the  enamel  organ,  and  at  an  early 
period  begins  to  assume  the  form  of  the  future  tooth-crown. 
Thus,  primarily,  the  papillae  for  the  incisors  will  have  their 
cutting-edges    ouriined    by   three    small    lobes,  each    of    which 


DEVELOPMENT  OF  THE  TEETH.  305 

represents  a  separate  point  of  calcification,  while  the  papillae 
for  the  molars  will  be  molded  according  to  the  number  of 
cusps  of  the  future  tooth,  a  small  tubercle  making  its  appear- 
ance for  each  cusp.  In  its  inception  the  dentin  papilla  is 
composed  of  cellular  tissue  identical  with  that  of  the  surround- 
ing parts.  The  growth  of  the  papilla  is  in  the  direction  of  the 
surface  ;  at  the  same  time  the  enamel  organ  forces  itself  more 
deeply  into  the  substance  of  the  parts,  not  only  overhanging 
the  coronal  extremity  of  the  papilla,  but  extending  about  and 
inclosing  its  lateral  walls.  Accompanying  the  growth  of  the 
papilla  is  a  rapid  change  in  its  structure,  becoming  more 
vascular  throughout,  and  its  peripheral  cells,  differentiating, 
form  the  essential  dentin  cells — the  odontoblasts.  This  layer 
of  cells  is  in  close  relation  to  the  enamel  cells  of  the  enamel 
organ,  the  combined  activity  of  the  two  finally  resulting  in  the 
calcification  of  the  tooth-crown.  The  dentin  papilla,  which 
eventually  becomes  the  tooth-pulp,  decreases  in  size  as  calcifi- 
cation proceeds  in  the  dentin,  all  additions  to  the  calcifying 
surface  taking  place  from  within  ;  while  the  enamel  organ  may 
be  said  to  increase  in  size,  the  calcific  action  in  the  enamel 
progressing  from  within  outward. 

The  Cells  of  the  Dentin  Papilla. — In  the  early  life  of  the 
dentin  germ,  the  cells  are  all  simple  embryonal  connective-tissue 
cells.  After  differentiation  takes  place  they  are  widely  scattered 
and  are  of  four  varieties:  spindle-shaped,  round,  stellate,  and 
the  elongated  or  club-shaped  odontoblasts.  None  of  these  are 
constant  in  location  except  the  layer  of  odontoblasts,  w^hich,  as 
has  been  said,  are  arranged  in  a  single  row  on  the  surface  of  the 
periphery  of  the  organ,  this  zone  being  classed  by  the  older  writers 
as  the  77ieinbrana.  ebo7-is.  Like  the  ameloblasts  of  the  enamel 
organ,  the  odontoblasts  do  not  make  their  appearance  until  the 
papilla  has  assumed  certain  proportions,  this  about  correspond- 
ing to  the  size  and  form  of  the  dentin  of  the  future  tooth.  Imme- 
diately beneath  the  layer  of  odontoblasts  appears  a  zone  almost 
devoid  of  cells.  This  is  followed  by  a  district  in  which  the  cells 
are  quite  numerous,  and  finally  when  the  central  portion  of  the 
papilla  is  reached  the  cells  are  again  few  in  number  and  widely 
scattered.     For  the  most  part  the  cells  in  the  interior  of  the 


3o6  ANATOMY. 

papilla  are  spindle-shaped  or  stellate,  having  rounded  nuclei 
about  which  there  is  a  small  amount  of  protoplasm  which  pene- 
trates the  intercellular  substance  by  numerous  hair-like  processes. 

The  Odontoblasts, — These  are  club-shaped  or  flask-shaped 
cells,  each  provided  with  a  large  nucleus  which  usually  assumes 
the  outline  of  the  enlarged  end  of  the  cell  which  is  directed 
toward  the  interior  of  the  papilla.  From  the  opposite  end  of 
the  cell,  or  that  directed  toward  the  enamel  organ,  and  in  close 
proximity  to  its  concave  surface,  one  or  more  protoplasmic 
processes  are  given  off.  These  persist  and  are  finally  encap- 
suled  within  the  calcified  dentin,  forming  the  dentinal  fibers. 
These  cells  are  very  closely  associated,  so  much  so,  in  fact,  that 
their  enlarged  extremities  are  almost  or  quite  in  actual  contact, 
more  or  less  space  existing  between  the  constricted  portion  of 
the  cells  as  they  pass  toward  the  surface. 

Germs  for  Permanent  Molars. — Reference  has  been  made 
to  the  fact  that  the  enamel  organs  for  the  deciduous  teeth  are 
o-iven  off  from  the  tooth-band  at  a  point  somewhat  distant  from 
its  free  margin,  so  that  the  tooth-band  is  continued  beyond  the 
primitive  enamel  germ,  this  free  margin  of  the  band  afterward 
generating  the  enamel  organ  for  the  succedaneous  tooth'.  As 
the  twelve  permanent  molars  are  not  succedaneous  teeth,  some 
other  means  must  be  provided  for  their  development. 

Opinions  of  various  writers  upon  this  subject  are  somewhat 
conflicting.  The  theory  is  advanced  by  some  that  as  the  jaw 
increases  in  length  the  tooth-band  and  lamina  primarily  pro- 
vided for  the  deciduous  teeth  are  extended  backward,  first  giv- 
ing off  a  bud  for  the  first  permanent  molar ;  at  a  somewhat 
later  period,  and  with  the  increase  in  the  growth  of  the  jaw,  an 
additional  bud  is  generated  for  the  second  molar,  the  third 
molar  being  provided  for  in  a  like  manner.  Another  theory, 
and  one  generally  accepted  as  correct,  is  that  the  cords  for  the 
permanent  molars  spring  individually  and  directly  from  the 
subepithelium.  There  may  be  found  an  exception  to  this  in 
the  case  of  the  first  permanent  molar,  which  sometimes  appears 
to  have  its  origin  from  the  distal  follicular  wall  of  the  second 
deciduous  molar.  Whatever  theory  be  accepted  in  regard  to 
the  genesis  of  these  permanent  organs,  the  process  of  develop- 


DEVELOPMENT  OF  THE  TEETH.  307 

ment  after  the  appearance  of  the  primary  bulb  or  enamel  germ 
is  identical  with  that  of  the  deciduous  teeth. 

The  Dental  Follicle,  or  Tooth-sac. — During  the  early 
life  of  the  tooth-germ,  both  the  enamel  organ  and  the  dentin 
papilla  are  differentiated  from  the  surrounding  parts  by  dissimi- 
larity of  structure  only,  but  as  development  proceeds,  a  more 
definite  separation  appears  between  the  tooth-generating  organs 
and  the  general  tissues  of  the  primitive  jaw,  this  separating 
medium  being  known  as  the  dental  follicle.  The  term  "follicle" 
is  only  one  of  a  number  applied  to  these  parts,  "dental  saccu- 


Calcified  Dent 


Dentin  Papilla, 
or  Tooth-pulp    ;v 

Wall  of  Follicle     Sfe^; 


Fig.   190. — Developing  Tooth  about  the  Fourth   Fetal  Month.     Appearance  of 
THE  Tooth-follicle. 


lus,"  "tooth-sac,"  and  other  appellations  being  employed  with 
equal  significance.  By  some  writers  it  is  customary  to  apply  the 
term  "  follicle  "  up  to  the  period  of  complete  closure,  the  term 
"sac"  or  "sacculus"  being  employed  after  that  time.  There 
appears,  however,  to  be  little  foundation  for  such  a  distinction, 
the  terms  being  synonymous.  Again,  the  follicle  is  frequently 
referred  to  as  meaning  the  sac  and  its  contents,  but  this  usage 
is  a  misapplication  of  the  term.  There  appears  to  be  no  well- 
founded  reason  why  the  follicular  wall   or  sac  should   not  be 


3o8  ANATOMY. 

referred  to  as  such,  regardless  of  the  formative  organs  within. 
As  to  the  development  of  the  tooth-foUicle,  it  appears  to  be  a 
generally  accepted  theory  that  at  a  very  early  period  there  is 
developed  from  the  base  of  the  papilla  cells  which,  differenti- 
ating, form  the  walls  of  the  follicle.  By  this  growth  of  cells  the 
periphery  of  the  papilla  is  first  surrounded,  and  this  step  is  soon 
followed  by  an  extension  of  the  cellular  structure  In  the  direction 
of  the  surface  epithelium,  to  the  deep  layer  of  which  the  cells 
become  firmly  attached,  and  in  so  doing  inclose  the  enamel 
organ,  which  hangs  like  a  hood  over  the   extremities  of  the 


Calcified  Dentin 
Enamel 


Outer  Enamel 
Epithelium 


Fig.  igi. — Development  of  Deciduous  Incisor,  from  Human  Fetus. — 
■    {After  Gysi.) 

papilla.  The  tissue  thus  formed  from  the  base  of  the  dentin 
germ  Is  continuous  with  and  similar  in  its  origin  to  the  pulp- 
substance.  The  primitive  tooth-germ,  during  the  formation  of 
the  follicular  wall,  is  found  swinging  in  a  membranous  pocket, 
being  supported  by  the  epithelial  band,  which.  In  turn.  Is 
attached  to  the  oral  epithelium  ;  but  as  the  walls  Increase  and 
completely  Inclose  the  germs,  which  Is  accomplished  iabout  the 
fourth  fetal  month,  the  epithelial  band  is  broken  and  the  second 
or  saccular  stage  of  tooth-development  Is  reached.  The  walls 
of   the   follicles   are   made   up   of  two   layers ;    the  outer  layer 


DEVELOPMENT  OF  THE  TEETH.  309 

is  dense  and  firm,  and  finally  becomes  the  dental  periosteum  ; 
the  inner  layer  is  thin,  frail,  and  in  the  recent  state  some- 
what transparent,  and  at  an  advanced  period  assists  in  the 
formation  of  the  cementum. 

Having  thus  briefly  described  the  primary  stage  of  tooth-de- 
velopment, the  careful  study  of  which  can  only  be  pursued  with 
the  aid  of  the  microscope,  we  will  now  pass  to  the  secondary  or 
saccular  stage.  By  the  introduction  of  a  number  of  illustrations, 
prepared  from  original  dissections  by  the  author,  this  phase  of 
the  subject  will  be  readily  comprehended. 

Before  continuing  the  subject  of  tooth-development,  it  will  be 
eminently  proper  to  briefly  describe  those  parts  directly  con- 
cerned in  the  process.     At  a  very  early  period  of  fetal  life  we 

That  Part  of  Cartilage  not  yet  Ossified. 


Head  of  Malle 


Handle  of  Malleus 


Ossified  MandiWe 

Fig.   192. — Developing  Mandible,  Three-month  Fetus. 


find  preparations  are  being  made  for  the  development  of  the 
maxillary  bones.  That  these  are  about  the  first  bones  to  be 
called  into  functional  activity  accounts  in  a  measure  for  their 
very  early  development.  The  osteoblastic  activity  in  the  inter- 
cellular substance  destined  to  become  the  inferior  maxilla  begins 
about  the  middle  of  the  second  month  of  fetal  life,  while  at  a 
somewhat  later  period  a  similar  action  takes  place  in  the  region 
of  the  superior  maxilla.  A  detailed  description  of  the  body  of 
these  bones  having  been  given  on  another  page,  it  will  not  be 
repeated  here,  but  to  that  portion  which  gives  lodgment  to  the 
tooth-germs,  and  which  in  a  measure  is  controlled  by  their  pres- 
ence, some  attention  must  be  given,  and  for  this  purpose  the 
mandible  will  principally  be  used. 


3IO 


Figure  192  represents  the  lingual  face  of  the  lower  jaw  after 
removal  from  a  three  months'  fetus.  Attached  to  it  is  the  re- 
maining portion  of  Meckel's  cartilage,  which  by  this  time  is 
much  wasted.  It  will  be  recalled  that  this  cartilaginous 
band  appears  in  the  mandibular  processes  before  the  begin- 
ning of  the  second  fetal  month,  being  formed  in  two  distinct 
halves,  the  free  ends  of  which  finally  unite  at  the  median  line, 
forming  a  continuous  support  or  framework,  about  which  ossifi- 
cation takes  place.  At  a  corresponding  period,  and  in  a  similar 
manner,  two  like  processes  are  thrown  out  for  the  superior 
maxillae,  but,  unlike  Meckel's  cartilage,  these  do  not  unite  at  the 
median  line,  but  stop  short  of  this,  the  space  thus  resulting 
being  provided  for    by  two  additional  processes,  which  shoot 

down  from  the  region  of  the  fore- 
head and  provide  for  the  develop- 
ment of  the  intermaxillary  bones. 
About  the  middle  of  the  second 
month  a  center  of  ossification  ap- 
pears in  the  neighborhood  of  the 
future  mental  foramen,  quickly  fol- 
lowed by  others  at  the  symphysis 
and  at  the  angle.  These  secondary 
centers  soon  unite  with  the  primary 
one,  and  by  the  end  of  the  second 
fetal  month  the  osseous  contour  of 
the  primitive  jaw  is  established. 
While  ossification  takes  place  in  the 
membrane  surrounding  Meckel's 
cartilage,  the  cartilage  itself  does  not  appear  to  be  directly 
concerned  in  the  process,  and  by  the  sixth  or  seventh  fetal 
month  the  mandibular  portion  completely  disappears,  while  that 
portion  near  the  tympanum  is  ossified  into  the  malleus.  That 
portion  of  the  bone  which  forms  above  Meckel's  cartilage  and 
the  inferior  dental  nerve  is  that  which  finally  gives  support  to 
the  tooth-germs.  This  cartilage  is  not  confined  to  the  human 
species,  but  is  the  common  heritage  of  reptiles,  rodents,  birds, 
and  fishes,  in  all  of  which  it  gives  support  to  the  developing 
lower  jaw. 


Fig.  193. — Evolution  of  the 
Mandible  from  the  Third  Fetal 
Month  to  Birth,  Two-thirds 
Actual  Size. 


DEVELOPMENT  OF  THE  TEETH. 


3" 


Figure  193  represents  the  evolution  of  the  mandible  from  the 
middle  of  the  third  fetal  month  to  the  time  of  birth.  It  will  be 
observed  that  during  this  interval  there  is  a  gradual  increase  in 
the  size  of  the  bone,  but  little  alteration  in  its  contour.  By  a 
constant  and  gradual  osseous  deposit  about  the  distal  extremity 
of  the  bone  its  length  is  increased  to  accommodate  the  additional 
teeth  as  they  make  their  appearance.  While  the  external  form 
of  the  bone  shows  but  slight  variation  during  this  period,  the 
internal  structure,  or  that  wherein  the  tooth-germs  lie,  is  under- 
going a  complete  transformation. 

Figure  194  is  illustrative  of  these  changes;  beginning  with 
a  simple  groove,  or  gutter,  into  which  the  tooth-follicles  hang, 
the  follicles  exerting  a  con- 
trolling influence  over  its 
form.  Next  comes  the  ap- 
pearance of  septa  between 
the  anterior  follicles,  which 
at  this  period  are  some- 
what irregularly  placed  in 
the  arch,  followed  in  a  few 
weeks  by  a  well-defined 
partition  between  the  cusp- 
ids and  molars,  until  final- 
ly, at  birth,  each  follicle  is 
inclosed  in  its  individual 
crypt,  with  the  single  ex- 
ception of  the  second  mo- 
lar, in  which  the  distal  septum,  or  that  which  is  to  separate  it 
from  the  permanent  first  molar,  has  not  yet  made  its  appear- 
ance. As  the  tooth-follicles  increase  in  size,  by  the  devel- 
opment of  the  teeth  within,  they  become  more  perfectly  in- 
closed in  the  bony  vaults,  the  sides  of  the  alveolar  walls 
arching  over  and  almost  completely  inclosing  the  developing- 
teeth.  Figure  195  shows  the  lower  jaw  of  a  seven-months-old 
child  embodying  the  condition  above  referred  to.  No  sooner 
have  the  crypts  grown  to  this  extent,  than  the  resorptive  action 
produced  by,  or  provided  for,  the  advancing  crowns  speedily 
results  in  their  downfall,  to  be  again  built  up  with  the  evolution 
of  the  permanent  teeth. 


Fig.  194.  —  Evolution  of  the  Mandible 
FROM  Third  Fetal  Month  to  Third  Month 
AFTER  Birth. 


About  the  first  visible  sign  of  preparation  for  tlie  develop- 
ment of  the  teeth,  other  than  that  made  apparent  by  dissection, 
may  be  observed  as  early  as  the  beginning  of  the  third  fetal 
month,  when,  upon  opening  the  cavity  of  the  mouth  and  looking 


nd  Periosteum  Lifted  Up 


Fig.   195. — Inferior  Maxilla  of  Seven-months -old  Child. 

upon  the  palate,  a  well-defined  infolding  of  the  epithelial 
eminence  will  be  seen.  In  figure  196  this  condidon  is  shown 
by  a  dissection  through  the  oral  cavity  of  a  four  months'  fetus, 
the  outer  fold  being  that  of  the  cheeks  and  lips,  while  within  are 
the  hard  palate  and  primitive  alveolar  ridge.    The  mouth  at  this 

Labiodental  Space  Labial  Fold 


Fig.  196. — Section  Through  the  Mouth  of  Four-month  Fetus. 


period  has  passed  the  rudimentary  state,  the  transverse  plates 
which  contribute  to  the  formation  of  the  hard  palate  having 
approached  each  other  until  the  oral  and  nasal  cavities,  hereto- 
fore existing  as  a  single  buccal  cavity,  have  become  separate 


DEVELOPMENT  OF  THE  TEETH. 


and  distinct.  The  infolding  of  the  oral  epithelium,  as  outlined 
on  the  summit  of  the  primitive  alveolar  ridge, — the  primitive 
dental  furrow,  so  called, — marks  the  position  of  the  tooth-band, 
from  which  are  given  off  the  incipient  tooth-bulbs.  For  the 
purpose  of  further  investigation,  a  dissection  of  these  parts  was 
made  and  the  maxillary  bones  removed,  after  which  they  were 
divested  of  their  fibrous  covering,  including  the  periosteum. 
That  portion  which  overlies  the  palatal  processes  was  readily 
lifted  in  one  sheet,  while  that  upon  the  facial  surface  was  sepa- 
rated at  the  median  line  and  stripped  independently  of  the  other 
(Fig.  197).  The  removal  of  these  tissues  is  readily  accomplished 
until  the  margins  of  the  partly 
formed  alveoli  are  reached.  Here 
the  periosteum  dips  down  into  the 
various  crypts,  and  serves  as  a 
lining  membrane  for  them,  and 
probably  contributes  fibres  to  the 
outer  layer  of  the  follicular  walls. 
After  advancing  thus  far,  the  de- 
tached tissues  may  be  grasped, 
and  by  careful  manipulation  the 
tooth-follicles  containing  the  form- 
ative organs  removed  from  their 
respective  vaults  and  turned  over 
for  examination. 

Figure  198  shows  the  result  of 
such  a  dissection.  On  the  left,  the  palatal  plates  and  alveolar 
walls  of  the  divested  bones  may  be  observed;  on  the  right,  is  the 
fibrous  covering,  which  has  been  turned  completely  over  after  re- 
moval from  the  bones,  having  firmly  attached  to  it  the  ten  tooth- 
follicles  for  the  deciduous  teeth.  If  this  dissection  be  made  with- 
out the  precaution  of  lifting  the  periosteum,  the  follicles  would 
not  cling  to  the  oral  membrane  with  sufficient  tenacity  to  permit 
of  their  ready  removal.  It  may  be  of  some  interest  to  note  that  at 
this  early  period  the  position  of  the  follicles  containing  the  germs 
for  the  lateral  incisors  is  that  which  the  tooth  is  forced  to  occupy 
up  to  and  frequently  beyond  the  eruptive  period,  being  crowded 
within  the  tooth-line  by  the  central  and  cuspid  follicles,  in  con- 


Oral  Mucous  Membrane 

Fig.  197. — Superior  Fetal 
Maxilla,  showing  manner  of 
Dissection  to  Expose  Tooth- 
sacs. 


sequence  of  which  the  lateral  crypts  are  thrown  well  into  the 
palatal  plates. 

Figure   199  represents  the   sacs  broken  down,   exposing  to 
view  the  dentin  papillae,  or  those  structures  destined  to  become 


Tooth-foUicles 


Maxilla  Oral  Mucous  Membrane 

Fig.  198. — Dissection  upon  Superior  Maxilla,  Fourth  Fetal  Month,  Exposing 
Tooth-follicles. 


the  tooth-pulps.  The  position  which  these  occupy  in  the 
illustration  is  exactly  the  reverse  from  that  which  they  assume 
when  in  position  in  the  follicle,  being  thus  reversed  that  a  better 
idea  of  their  shape  may  be  obtained.  Prior  to  the  twelfth  or 
thirteenth  week  of  fetal  life  this  incipient  bulb  or  papilla  is  with- 


Oral  Mucous  Meuibr; 


e,  Turned  Completely  Ov 
Fig.  199. 


out  definite  form  ;  but  by  the  latter  period  each  papilla  begins 
to  assume  the  contour  of  the  future  tooth-crown,  as  faintly  out- 
lined in  the  illustration,  those  of  the  incisors  presenting  the 
angular  form  of  the  future  cutting-edge,  the  cuspids  that  of  the 


DEVELOPMENT  OF  THE  TEETH. 


31S 


single  cone,  while  the  coronal  extremities  of  the  molars  are 
represented  by  outlines  corresponding  to  the  future  cusps  and 
marginal  ridges.  Besides  the  dentin  papilla,  there  is  contained 
within  the  follicular  walls  the  organ  which  later  on  is  productive 
of  the  enamel,    but  which  up  to  this   time  has    been  actively 

Oral  Mucous  Membrane  and  Periosteum  Dissected  from  Mandible 


Attachment  of  Follicle  to  Oral  Membrane  (Enlarged  One-third) 

Fig.  200. — M.a.nner  of  Dissection  to  Expose  Tooth-sacs. 

engaged  in  molding  the  tooth-form  as  outlined  by  the  papilla. 
To  return  to  the  tooth-follicle,  the  dissection  in  this  instance 
being  upon  the  lower  jaw  of  a  four  months'  fetus.  Figure  200 
shows  the  mandible  removed  and  the  dissection  carried  to  the 

Tooth-follicles      Oral  Membrane 


Fig.  201.— Tooth- follicles  Removed  from  Mandible,  Fourth  Fetal  Month. 


point  at  which  the  follicles  may  be  lifted  from  their  bony  encase- 
ments, this  being  accomplished  by  an  incision  along  the  base  ot 
the  bone,  followed  by  a  stripping  of  the  membrane  first  from 
the  facial  and  then  from  the  lingual  side  of  the  bone.  When 
these  two  flaps  reach  the  margin  of  the  crypts,  they  are  firmly 


3i6  ANATOMY. 

grasped  and  the  follicles  removed  from  their  sockets,  as  illus- 
trated in  figure  201. 

At  the  beginning  of  the  saccular  stage  of  development  the 
form  of  the  future  tooth-crown  is  well  outlined  by  the  dentin 
papilla,  which  in  figure  202  is  brought  into  view  by  a  dissection 
of  the  walls  of  the  follicles  shown  in  figure  201  without  breaking 
the  attachment  existing  between  the  two.  As  in  the  case  of  the 
follicle,  much  confusion  of  terms  in  regard  to  this  structure 
exists,  the  "dentin  bulb,"  the  "pulp,"  "dentin  germ,"  and  the 
"  papilla  "  being  used  ad  libitum,  without  apparent  regard  for  the 
structural  changes  which  are  continuously  affecting  the  organ. 
It  will,  therefore,  be  proper  to  simplify  this  conglomeration  of 

Space  Occupied  by    Oral  Mucous 
Enamel  Organ  Membrane 


Follicular  Wall 


Dentin  Papillae 

to  Beginning 
of  Calcification 


Fig.  202. — Tooth-follicles  shown  in  Figure  201  Opened. 


terms  by  a  classification  appropriate  to  the  various  stages  of 
development.  The  term  "dentin  papiUa"  will  best  describe  this 
part  of  the  tooth-germ  up  to  the  time  of  beginning  of  calcifica- 
tion, subsequent  to  which  the  term,  "pulp  "  should  be  employed. 
As  previously  stated,  the  enamel  organ,  until  this  period,  has 
been  principally  devoting  its  energies  to  the  molding  of  the 
tooth-form,  and  it  is  not  until  this  model,  as  represented  in  the 
papilla,  is  complete  that  the  process  of  calcification  begins. 
About  the  fourth  fetal  month  preparations  for  the  calcification 
of  the  deciduous  teeth  are  begun  by  the  development  of  the 
odontoblastic  cells  for  the  dentin,  which  first  make  their  appear- 
ance on  the  periphery  of  the  dentin  papilla,  the  summits  of  the 


DEVELOPMENT  OF  THE  TEETH.  317 

various  cusps  in  the  molars  and  the  future  cutting-edges  of  the 
incisors  being  first  affected.  This  phenomenon  is  soon  followed 
by  the  appearance  of  the  ameloblastic  cells  for  the  enamel, 
which  establish  themselves  in  the  internal  epithelial  layer  of 
the  enamel  organ. 

Figure  203  is  prepared  from  a  dissection  made  in  a  manner 
similar  to  that  shown  in  figure  199,  but  at  a  period  about  a 
month  later,  being  from  the  superior  maxilla  of  a  five  months' 
fetus.  The  dissection  shows  the  extent  of  calcification  at  this 
period,  which  process  also  defines  the  position  of  the  odonto- 
blastic cells  upon  the  extremity  of  the  papillse.  In  the  incisors 
(one  of  which  was  lost  in  the  preparation  of  the  specimen)  the 

Calcified  Caps        Tooth-pulp  Maxilla: 


Fig.    203 


dentin  may  be  seen  capping  the  cutting-edges.  The  cuspids  in  this 
subject  have  not  yet  begun  to  calcify,  although  it  is  not  unusual 
to  find  the  cusp  of  this  tooth  receiving  its  lime-salts  at  this  early 
period.  In  the  molars  the  summits  of  the  various  cusps,  as  well 
as  a  portion  of  the  various  ridges  descending  therefrom,  are 
undergoing  the  change  produced  by  the  impregnation  of  the 
lime-salts.  It  is  quite  probable  that  these  delicate  caps  are  at 
this  time  composed  of  dentin  alone,  the  calcoglobulin  which 
precedes  the  enamel  calcification  forming  somewhat  later. 
From  this  time  forward  the  pulp  undergoes  a  gradual  trans- 
formation as  to  size  and  form,  and  there  is  likewise  a  change  in 
its  cellular  construction  on  those  parts  adjacent  to  the  calcific 
action.     While  the  outline  of  the  pulp  is  gradually  changing,  its 


31 8  ANATOMY. 

original  form  is  permanently  recorded  upon  the  periphery  of  the 
dentin  cap,  which,  when  once  formed,  is  immutable,  all  additions 
taking  place  from  within. 

Figure  204  illustrates  the  result  of  a  dissection  upon  the  lower 
jaw  of  the  same  subject,  disclosing  practically  the  same  condi- 

Calcified  Dentin        Oral  Mucous  Membrane 


Space  Occupied  by 
Enamel  Organ 


Fig.  204. — Tooth-follicles   Opened,  Exposing   Dentin   Papillae  and  Beginning  of 
Calcification,  Fifth  Fetal  Month. 

tions,  with  the  exception  of  the  sac  containing  the  developing 
cuspid,  which  was  found  with  a  slightly  calcified  cap  of  dentin. 
This  slight  variation  between  the  development  of  the  upper  and 
lower  teeth  is  one  that  is  present  in  nearly  every  instance,  the 
latter  being  somewhat  in  advance  of  the  former. 

In  figure  205  the  tooth-pulps,  with  their  primitive  cappings 


Fig.  205. 


of  dentin,  have  been  removed  from  the  follicles,  and  a  better 
opportunity  of  studying  the  relations  between  the  two  parts  is 
presented.  By  the  conversion  of  the  coronal  extremities  of  the 
dentin  papilla  into  odontoblasts,  and  their  active  calcification, 
some  positive  union  between  the  two  parts  might  be  expected. 


DEVELOPMENT  OF  THE  TEETH.  319 

On  the  contrary,  the  dentin  caps  are  readily  removed,  leaving 
the  pulp  beneath  without  the  slightest  rupture,  so  that  we  find 
calcification  is  not  a  secretory  or  excretory  metamorphosis,  but 
that  the  change  takes  place  within  the  substance  of  the  papilla 
itself,  whereby  it  is  altered  from  an  organic  to  an  inorganic 
substance. 

The  next  dissection  was  one  upon  the  mandible  of  a  six 
months'  fetus.  Figure  206  shows  the  tooth-follicles  removed 
from  the  partially  formed  bony  crypts  in  which  they  have  been 
incased.  At  an  early  period  of  fetal  life,  and  at  a  time  prior  to 
the  completion  of  the  tooth-follicles,  there  is  deposited  beneath 
the  tooth-germs  a  thin  layer  of  bone,  which  at  once  begins 
to  assume  the  form  of  the  partially  developed  follicular 
walls.     As  the  growth  of  the  follicle  proceeds,  there  is  a  corre- 


Liiigual  Surface  of  Mandible 
Fig.    206. 


sponding  increase  in  the  osseous  deposit,  the  alveolar  walls 
extending  about  and  accommodating  themselves  to  the  mem- 
branous sacs.  Thus  we  find  in  this  portion  of  the  maxillary 
bones  a  feature  peculiar  to  itself — that  of  a  condnuous  trans- 
formation from  its  earliest  inception  to  the  adult  period,  first 
developing  about  the  temporary  tooth-sacs  and  completely 
incasing  them,  which  is  speedily  followed  by  complete  resorption 
of  the  walls,  again  followed  by  a  rebuilding  during  the  evolution 
of  the  permanent  teeth,  and  again  swept  away  with  the  loss  of 
these  organs.  Figure  207  illustrates  the  opposite  side  of  the 
same  jaw,  with  its  outer  or  facial  plate  removed,  together  with 
the  intervening  septa.  The  follicles  are  opened,  and  the  extent 
of  calcification  at  this  period  (six  months)  made  apparent.     The 


320  ANATOMY. 

pulps  and  calcified  caps  are  approximately  as  found  when 
dissected,  save  a  slight  settling  of  all  the  parts.  The  incisors 
have  calcified  to  about  one-third  their  full  coronal  length  ;  the 
unicusped  contour  of  the  cuspid  has  been  established,  as  shown 
by  the  deposit  of  the  lime-salts  upon  its  summit,  and  the 
upbuilding  of  the  mesial  and  distal  cutting-edges.      The  first 

Follicular  Wall  Calcified  Caps        Oral  Mucous  Membrane 


Dentin  Papilla, 
First  Perma- 
nent Molar 


Tooth-pulps 

Fig.    207. 

molar  has  about  completed  its  occlusal  surface,  and,  while  the 
cusps  of  the  second  molar  are  nearing  completion,  there  is  a 
lack  of  union  in  the  central  and  distal  fossse.  Immediately 
posterior  to  the  second  deciduous  molar,  the  sac  containing 
the  formative  organs  for  the  first  permanent  molar  is  shown 
opened,  exposing  to  view  the  dental  papilla,  which  at  this  early 


M   **    U^ 


period  has  assumed  the  form  of  the  future  tooth-crown,  and 
calcification  is  about  to  begin.  Figure  208  illustrates  the  extent 
of  calcification  in  the  deciduous  teeth  at  the  sixth  fetal  month. 
As  the  growth  of  the  teeth  proceeds,  it  will  be  observed  that 
the  angularity  which  originally  accompanied  the  calcifying  caps 
of  the  molars    is    gradually  disappearing,   as    is    also    the   tri- 


DEVELOPMENT   OF  THE  TEETH.  321 

tubercular  form  of  the  incisors  and  cuspids,  this  change  being 
brought  about  by  the  deposition  of  enamel  to  the  parts. 

Figures  209  and  210  show  the  calcified  caps  removed  from 
the  pulps,  and  so  arranged  that  both  their  external  and  internal 
anatomy  may  be  studied.  Prior  to  this  time  there  has  been  but 
little  alteration  in  the  form  of  the  dentin  pulp,  only  a  gradual 
decrease  in  its  size  being  noted ;  but  now  we  find  it  being 
divested  of  many  of  its  angles,  particularly  those  which  origi- 


FiG.  2og. 

nally  served  as  a  basal  form  for  the  coronal  extremities  of  the 
future  tooth.  With  the  disappearance  of  these  the  concavity 
within  the  cap  is  slowly  assuming  the  form  of  the  future  pulp- 
chamber. 

That  a  better  understanding  of  the  saccular  stage  of  tooth- 
development  might  be  had,  a  transverse  section  was  made 
through  the  molar  follicles,  as  shown  in  figure  211.     In  this  the 


attachment  of  the  follicular  walls  to  the  deep  epithelial  layer  is 
visible,  while  within  the  walls  is  the  enamel  organ,  the  calcified 
dentin,  and  the  tooth-pulp.  As  previously  stated,  the  enamel 
organ  is  seen  suspended  above  and  forming  a  hood-like  invest- 
ment to  the  calcifying  structure.  This  organ  not  only  over- 
hangs the  occlusal  surface  of  the  tooth-crown,  but  completely 
envelops  the  sides  of  the  calcified  cap  and  dentin  pulp.  Previous 
to  the  beginning  of  calcification  the  enamel  oroan  is  in  close 


proximity  to  the  dentin  papilla,  the  original  form  of  the  latter 
being  represented  by  the  calcified  dentin. 

We  have  now  arrived  at  that  period  of  fetal  existence  when  it 
is  possible  to  study  the  macroscopic  development  of  the  perma- 


Enamel  Organ 


Follicular  Wall 


Tooth-pulp        Floor  of  Tooth-follicle 

Fig.  211. — Dissection  Showing  Pulp,  Calcified  Cap,  and  Enamel  Organ. 


nent  first  molar.  Figure  212  represents  a  section  of  the  lower 
jaw  of  a  six  months'  fetus,  and  displays  not  only  the  sacs  of  the 
deciduous  teeth,  but  also  that  of  the  permanent  first  molar.  In 
most  respects  the  evolution  of  this  tooth  is  similar  to  that  of  the 
temporary  organs,  having  its  origin  from  the  deep  epithelial 
layer,  either  directly  or  by  continuation  of  the  tooth-band 
backward.     Preparations  for  its  growth  are  begun  as  early  as 

the  third  fetal  month,  at  which 
Oral  Mucous  Membrane  time  the  enamel  organ  is  given 

off,  and  thereafter  the  develop- 
mental process  is  identical  with 
that  of  the  deciduous  teeth. 

There    is   one   structure,  how- 
ever,   intimately  connected    with 
the   development  of  the  perma- 
nent teeth  not  found  in  connec- 
FiG.  212.  tion  with  the  deciduous  organs — 

the  gubernacuhun,  or  leading  cord. 
Figure  213  represents  another  section  of  a  six  months'  fetal 
mandible,  with  the  dentin  papilla  for  the  permanent  first 
molar  turned  out  from  the  follicle  after  being  rolled  from  its 
bony  incasement.     Attached  to  the  apex  of  the  tooth-sac  (which 


DEVELOPMENT  OF  THE  TEETH.  323 

has  been  turned  back),  and  leading-  from  it  to  the  epithelium  of 
the  jaw,  is  the  gubernaculum.  This  fibrous  structure  was  at 
one  time  thought  to  be  directly  concerned  in  the  development 
of  the  tooth.  Although  this  is  denied  at  present,  little  is 
said  in  regard  to  its  function,  but  it  undoubtedly  serves  the 
purpose  of  directing  the  tooth  to  that  position  which  it  should 
occupy  in  the  jaw,  and  where  the  least  resistance  to  its  eruption 
is  formed  by  the  foramen  which  the  cord  has  established.  Each 
of  the  permanent  teeth  is  provided  with  a  similar  membranous 
cord,  an  illustration  and  more  complete  description  of  which  will 
follow  later  on. 

We  have  now  arrived  at  a  period  when  the  subject  under 

Oral  Mucous  Membrane        Gubernaculum 


II 

i^^ 

Dentin  Papilla 
Fig.  213. 


consideration  naturally  becomes  of  deeper  interest.  I  refer  to 
that  time  when  the  being  changes  from  a  complex  dependent 
condition  to  one  of  self-providing  independence.  Previous  to 
the  time  of  birth  the  teeth  appear  to  be  but  little  disturbed  by 
certain  morbid  conditions  which  might  be  present  in  the  parent, 
and  from  their  earliest  inception  up  to  this  period  their  develop- 
ment proceeds  with  but  little  interruption  and  with  much  regu- 
larity. Figure  214  illustrates  the  condition  of  the  deciduous  teeth 
at  birth ;  the  central  incisors  are  calcified  externally  to  the  cer- 
vical line,  the  lateral  incisors  to  a  point  corresponding  to  the 
summit  of  the  palatocervical  ridge  ;  the  cuspids  have  advanced 
somewhat  beyond  the  angles  of  the  crown,  while  the  molars  have 
their  crowns  calcified  to  about  one-half  their  completed  length. 


With  all  this  progress  as  represented  by  the  external  contour 
of  the  tooth-crowns,  the  internal  form  appears  to  be  somewhat 
slow  in  assuming  the  shape  of  the  future  pulp-chamber.  From 
the  beginning  of  the  saccular  stage  of  development  up  to  the 
time  of  birth  there  is  but  little  increase  in  the  diameter  of  the 
tooth-sac,  but  there   occurs  a   gradual    increase  in  its  length. 


Fig.  214. — Deciduous  Teeth  at  Birth.     (Reflected  picture.) 

Figure  215  shows  the  mandible  from  a  child  one  week  old,  with 
the  greater  part  of  the  external  or  facial  surface  of  the  bone 
removed,  exposing  not  only  the  sacs  containing  the  developing 
deciduous  teeth,  but  also  that  of  the  permanent  first  molar. 
The  relation  of  the  sacs  to  the  inferior  dental  canal  is  apparent, 
as  well  as  the  firm  attachment  of  the  follicular  walls  to  the  oral 
membrane.     In  figure  216  the  tooth-sacs  have  been  dissected 


Fig.  215. 

and  the  pulps  removed  from  the  calcified  caps,  presenting  an 
additional  illustration  of  the  amount  of  dentin  deposit  at  this  age- 
To  further  illustrate  the  size  and  form  of  the  pulp  as  compared 
with  the  calcified  cap  at  birth,  a  transverse  incision  was  made 
through  the  left  superior  maxilla,  at  a  point  corresponding  to 
the  base  of  the   pulp,  as  shown  in  figure  217.     The   calcified 


DEVELOPMENT  OF  THE  TEETH 


325 


parts  remain  in  position  resting  against  the  remaining  portion 
of  the  enamel  organ,  while  the  pulps  are  dislodged  and  may  be 
observed  resting  upon  the  incised  surface.     In  this  illustration 


Dentin  Papilla  of 
Permanent  First 
Molar 


the  dentin  papilla  for  the  first  permanent  molar  is  also  seen, 
being  supported  by  the  walls  of  the  follicle,  which  in  turn  are 
attached  to  the  oral  epithelium  by  the  gubernaculum.     It  may 


Calcified  Caps,  Resting  against 
Enamel  Organ 


Tooth-pulps  Rolled  Out 


Section  of  Superior  Ma 


Pulp  of  Permanent  First  Molar 
Fig.    217. 


also  be  noted  that  those  parts  of  the  pulp  corresponding  to  the 
cusps  and  marginal  ridges  show  a  decided  convergence  of  the 
surface  toward  the  center. 


326 


ANATOMY. 


Reference  has  been  made  to  the  formation  of  the  follicular 
walls  by  a  differentiation  of  cells,  which  at  an  early  period  are 
given  off  from  the  base  of  the  papillae,  and  to  the  continuity  of 
the  two  structures.  Figure  218  was  prepared  for  the  purpose 
of  showine  these   intimate  relations.     The    sides   of   the    sacs 


Dentin  Papilla  for 

i 

1 

Molar 

Papilla 
Calcified  Cap 

Walls  of  Tooth- 

f5 

were  opened  and  turned  back  ;  the  calcified  tooth-caps  with  pulps 
in  position  were  grasped  and  given  several  revolutions,  thus 
twisting  the  remaining  portion  of  the  walls,  the  floor  of  which  is 
seen  as  a  continuous  structure  given  off  from  the  pulp  and  con- 
necting it  with  the  epithelium  of  the  jaw. 


PREPARATIONS  FOR  THE  DEVELOPMENT  OF  THE 
PERMANENT  TEETH. 
A  little  before  the  time  of  birth  sufficient  advance  has  been 
made  in  the  development  of  the  permanent  teeth  to  permit  a 
study  of  their  relations  with  the  temporary  organs.  The  early 
preparation  for  the  growth  of  these  teeth  was  for  a  long  time 
a  subject  of  much  controversy,  some  writers  advancing  the 
theory  that  the  buds  for  the  permanent  teeth  were  produced  or 
given  off  from  the  sacs  of  the  temporary  teeth  ;  others  con- 
tending that  the  cords  were  derived  from  the  remnants  of  the 


DEVELOPMENT   OF   THE   PERMANENT   TEETH.  327 

primitive  cords  immediately  after  their  rupture.  The  theory 
now  generally  accepted  is  that  the  cord  is  given  off  from  the 
primitive  cord  at  a  point  in  close  proximity  to  its  attachment  to 
the  deciduous  enamel  organ.  This  can  only  apply  to  those 
teeth  which  are  succedaneous,  and,  therefore,  does  not  include 
the  permanent  molars,  as  heretofore  stated.  Whatever  theory 
be  accepted  in  regard  to  the  genesis  of  the  permanent  teeth, 
there  can  be  no  mistake  in  regard  to  that  part  of  the  process 
which  we  are  permitted  to  ocularly  investigate.  I  shall,  there- 
fore, proceed  to  describe  the  position  and  contents  of  the  per- 
manent tooth-sacs  at  birth.  Figure  219  presents  the  result  of  a 
vertical   dissection    through  the   superior   maxillary  bones,   the 


Guberiiaculuni        Tooth-sac  of  Receding  Permanent  Tooth 

Fig.  219.— Section  through  Superior  Maxill.«,  Sixth  Month  after  Birth. 

inner  surface  of  the  right  maxilla  being  exposed  to  view. 
Many  of  the  frail  processes,  particularly  those  entering  into 
the  construction  of  the  nasal  cavities,  were  lost  during  the 
preparation  of  the  specimen,  the  whole  purpose  of  the  dissec- 
tion being  to  show  the  sac  of  the  permanent  incisor  and  its 
relationship  to  its  predecessor.  By  this  time  calcification  in  the 
deciduous  incisor  has  so  far  advanced  that  the  contour  ot  the 
tooth-crown  may  be  plainly  outlined  through  the  walls  of  the 
sac.  Resting  against  the  palatal  concavity  of  the  crown  of  the 
temporary  incisor  is  the  sac  containing  the  formative  organs 
for  its  permanent  successor.  This  permanent  tooth-sac  does 
not  long  remain  in  such  close  proximity  to  the  deciduous  tooth- 


328  ANATOMY. 

crown,  for  as  the  latter  advances  toward  the  surface  of  the  gum 
the  former  recedes  and  is  soon  inclosed  in  a  separate  crypt, 
which,  were  it  not  for  the  gubernacular  foramen,  w^ould  com- 
pletely inclose  it.  Figure  220  represents  a  section  of  the  left 
superior  maxilla,  introduced  at  this  point  for  the  purpose  of 
showing  the  position  of  the  foramina  for  the  gubernacula. 
These  may  be  observed  immediately  posterior  to  the  incisor 
and  cuspid  teeth.  At  birth  these  foramina  do  not  exist  as  such, 
the  partially  formed  vaults  containing  the  sacs  for  the  permanent 
teeth  appearing  as  an  extension  of  the  temporary  crypts  in  a 
palatal  direction  ;  but  as  the  temporary  teeth  advance  and  the 

Foramina  for  Gubernacula 


permanent  teeth  recede,  the  roof  of  the  crypt  is  completed,  and 
the  foramen  established  by  the  presence  of  the  gubernaculum. 
This  extension  of  the  temporary  crypts  is  more  clearly  demon- 
strated in  figure  221,  which  represents  one  side  of  the  lower 
jaw  at  birth  with  the  partially  calcified  deciduous  teeth  in  posi- 
tion in  the  bone.  Suspended  above  this  is  the  gum,  which  has 
been  dissected  from  the  bone,  having  attached  to  its  under 
surface  the  sacs  for  the  permanent  teeth.  Those  for  the 
incisors  are  particularly  well  defined  and  their  place  of  lodg- 
ment in  the  bone  readily  noted.     In  the  case  of  the  cuspid,  both 


DEVELOPMENT   OF   THE   PERMANENT   TEETH.  329 

the  deciduous  tooth  and  the  permanent  tooth-sacs  are  in  posi- 
tion in  the  crypt.  The  cords  which  support  the  incisors  are 
somewhat  lengthened  from  the  weight  of  the  sacs,  the  guber- 
nacula  not  assuming  this  thread-like  form  until  the  permanent 
sacs  have  further  receded. 

Figure    222    shows    the    result    of  a  dissection    upon  these 


;  for  Permanent  In 


Deciduous  Teelh  in  Bony  Crypts 
Fig.  221. 


permanent  tooth-follicles.  The  dentin  papillae  of  the  various 
teeth  are  seen  in  a  reversed  position,  with  the  follicular  walls 
attached  to  their  bases.  At  this  period  the  papillae  for  the 
permanent  incisors  may  be  compared  to  the  tail  of  a  fish,  being 
perfectly  transparent  over  its  free  extremity,  which  feature  is 


Oral  Mucous  IMembrane 


Follicul 
Wrc 

arWall.Tur 
ug  Side  Out 

Denti 

led 

-^l 

r"^ 

Papilla  for  Perma 
Molar 

,Pap 

ilia:  for  Permj 

nent  Teeth,  Upside  Down 
Fig.  222. 

gradually  lost  as  its  thickened  base  is  approached.  The  fish-tail 
appearance  is  further  represented  by  the  division  of  the  free 
extremity  into  three  distinct  parts,  each  of  which  provides  a 
separate  point  of  calcification.  The  cuspid  papilla  is  missing, 
and  the  bicuspids  have  advanced  little  beyond  the  form  of  the 


primitive  bulb.  The  first  molar  is  shown  with  the  full  diameter 
of  the  crown  represented  by  the  pulpal  mass,  and  the  tips  of  the 
cusps  are  already  beginning  to  take  on  the  calcific  action. 


Tooth-sacs  oi  Permanent  Teeth 


Lingual  Surface  ol  Mandible 

Fig.  223. 


Toolh-sacs  of  Deciduous  Teeth 


Tooth-sac  of  Permanent  Tooth 


Fig.  224.— Same  as  Figure  223,  Except  on  Upper  Jaw. 


A  further  illustration  of  the  progress  of  the  development  of 
the  permanent  teeth  and  their  relation  to  the  deciduous  organs 
may  be  seen  in  figure  223,  the  dissection  in  this  instance  being 


DEVELOPMENT   OF   THE   PERMANENT   TEETH. 


331 


upon  the  lower  jaw  of  a  one-month-old  child.  The  membrane 
has  been  lifted  from  the  bone  with  the  tooth-sacs  attached  to  it. 
Immediately  posterior  to  the  sacs  containing  the  crowns  of  the 
temporary  incisors  and  cuspids  are  those  for  their  corresponding 
successors,  the  papillae  of  which  have  already  assumed  the 
tubercular  outline  of  the  future  cutting-edge.  While  the  per- 
manent tooth-sacs  are  distinctively  independent  pouches,  there 
appears,  nevertheless,  to  be  a  well-established  fibrous  connec- 
tion existing  between  the  outer  layer  of  the  two  follicular  walls. 
This  fibrous  union  is  gradually  broken  as  the  permanent  sacs 
recede  and  become  incased  in  their  own  vaults. 

While  the  permanent  tooth-sacs  are  generally  referred  to  as 
"  receding,"    it   is   a  question    if    this    term    is    fully    justified. 


Tooth-sacs  of  Decidu 


Periosteum  of  Hard  Palate 

Fig.  225. — Tooth-follicles   for  Deciduous  and   Permanent  Teeth,  Three  Months 
AFTER  Birth. 


While  the  follicles  do  not  remain  in  close  relation  with  their 
predecessors,  the  change  in  the  relative  position  of  the  two  is 
principally  brought  about  by  the  advance  in  the  deciduous  sacs, 
this  forward  movement  being  accompanied  by  a  marked  growth 
of  the  bone  in  the  direction  of  the  future  alveolar  ridge,  thus 
leaving  the  permanent  tooth-sacs  well  buried  in  the  substance 
of  the  jaw. 

Figure  225  represents  the  result  of  a  dissection  upon  the 
superior  maxillae  of  a  three-months-old  child.  The  mucous  mem- 
brane covering  the  hard  palate,  together  with  the  periosteum. 


332 


ANATOMY. 


has  been  dissected  from  the  bones  and  turned  over  for  exam- 
ination. The  tooth-follicles  for  all  the  deciduous  teeth,  as  well 
as  those  of  the  succedaneous  permanent  organs,  may  be  ob- 
served firmly  attached  to  the  fibrous  tissue.  The  permanent 
incisor  sacs  at  this  age  are  almost  equal  in  size  to  those  of  their 
predecessors,  and  the  dentin  papillae  within  possess  a  mesio- 
distal  diameter  almost  equal  to  those  of  the  calcified  temporary 
caps.  The  sacs  containing  the  germs  for  the  permanent  cuspids 
and  bicuspids  are  somewhat  diminutive,  but  the  enamel  organs 
within  are  already  molding  the  contour  of  the  future  tooth- 
crowns  upon  the  dentin  papillae. 

By  the  beginning  of  the  second  month  after  birth  calcification 
in  the  crowns  of  all  the  deciduous  teeth  is  about  complete,  and 
preparation  for  the  growth  of  the  roots  is  under  way.     While 


at  this  period  the  tooth-crowns  may  be  said  to  be  almost  com- 
pletely calcified,  this  does  not  apply  to  the  interior  of  the 
crowns,  the  deposit  of  dentin  internally  being  a  continuous 
process,  resulting  in  a  gradual  reduction  in  the  capacity  of  the 
pulp-cavity.  It  is  also  quite  probable  that  the  enamel  organ  is 
somewhat  active  up  to  the  eruptive  period,  and,  if  this  be  true, 
the  enamel  covering  of  the  crown  is  not  complete  until  this 
time.  Whatever  be  the  condition  in  the  crowns,  the  time  for 
the  formation  of  the  roots  has  arrived,  and  it  is  principally 
through  the  activity  of  the  tooth-pulp  that  they  are  generated. 
We  have  seen  that  the  contour  of  the  tooth-crown  was  first 
molded  upon  the  dentin  papilla ;  so  it  is  with  the  tooth-root :  by 
a  gradual  elongation  of  the  sac,  accommodations  are  afforded 
the    tooth-pulp    for    a    corresponding    growth.     As    the    pulp 


DEVELOPMENT   OF  THE   PERMANENT   TEETH.  333 

lengthens  out  toward  the  future  apex  of  the  root,  it  is  molded 
to  the  root-form,  and  calcification  takes  place  by  the  generation 
of  odontoblastic  cells  upon  the  periphery  of  this  organic  root- 
form. 

While  the  process  of  root-formation  in  the  single-rooted  tooth 
may  be  readily  comprehended,  the  bifurcation  or  trifurcation 
of  the  molar  roots  presents  a  complication  which  calls  for  special 
reference.  Figure  226  will  assist  in  e.xplaining  this  phenome- 
non. In  the  illustration  three  deciduous  molar  crowns  are 
shown,  two  of  which  are  incased  in  their  tooth-sacs,  the  third 
being-  stripped  of  this  membrane.  The  view  is  directly  upon 
the  base  of  the  tooth-sacs,  immediately  beneath  which  is  the 
base  of  the  pulp.  Up  to  this  period  the  odontoblastic  cells  have 
been  generating  about  the  occlusal  surface  and  lateral  walls  of 
the  crown  only,  but  now  an  accumulation  of  these  cells  is  to  be 
found  upon  the  base  of  the  pulp,  lining  up  in  the  position  of 
the  future  root-walls.  This  structural  change  is  faintly  out- 
lined in  the  illustration.  By  this  inward  extension  of  the  odon- 
toblastic cells  from  various  points  about  the  margins  of  the 
pulp,  and  their  union  near  the  center  of  the  mass,  provision  is 
made  for  the  calcification  of  the  various  roots,  which  process  is 
considered  separately  by  an  extension  and  molding  of  the  pulp 
into  two  or  more  divisions. 

Calcification  of  the  Cementum. — While  the  dentin  of  the 
root  is  derived  from  the  tooth-pulp,  the  external  covering  of  the 
root  (the  cementum)  is  generated  from  another  source.  In 
every  respect  cementum  is  closely  allied  to  bone,  and  we  find 
its  development  provided  for  in  a  similar  manner.  As  stated  in 
another  part  of  this  chapter,  the  tooth-sac  is  made  up  of  an 
outer  and  an  inner  layer,  both  of  which  are  rich  in  blood- 
vessels. These  membranous  walls  continue  to  invest  the  roots 
of  the  teeth  during  their  upbuilding.  The  outer  layer  of  the  sac 
remains  as  a  permanent  structure  placed  between  the  root  and  the 
alveolar  walls,  forming  the  alveolodental  membrane,  while  upon 
the  surface  of  the  inner  layer  osteoblasts  (cementoblasts)  are  de- 
veloped, which  are  speedily  converted  into  bone  or  cementum.  In 
this  process  the  tooth-root  may  be  compared  to  one  of  the  long- 
bones  of  the  body,  and  the  development  of  the  cementum  con- 


sidered  under  the  head  of  Subperiosteal  Ossification.  The  only 
variation  to  be  observed  between  this  and  subperiosteal  de- 
velopment of  bone  is  in  the  presence  of  a  single  Haversian 
canal  (as  the  pulp-cavity  may  be  considered),  and  even  this  dif- 
ference is  sometimes  overthrown  by  small  canals  running  at 
right  angles  to  the  pulp.  These  small  canals  are  generally 
found  near  the  apex  of  the  root,  at  which  point  the  cementum 
is  the  thickest.  Like  the  enamel  cap  of  the  tooth-crown,  the 
cementum  is  deposited  upon  the  surface  of  the  dentin  of  the 
root,  thus  increasing  its  diameter. 

Eruption  of  the  Teeth. — Up  to  this  time  no  reference  has 
been  made  to  that  process  by  which  the  teeth  burst  forth  from 
their  bony  incasements,  and,  penetrating  the  mucous  membrane, 
make  their  appearance  in  the  mouth.  Attention  has  been 
called  to  the  growth  of  the  bone  about  the  tooth-follicles, — first 
forming  beneath  them  as  an  open  gutter,  next  surrounding  their 
lateral  walls  and  inclosing  each  follicle  in  a  separate  compartment, 
and  finally  each  tooth  becoming  more  completely  enveloped  by 
an  arching-over  of  the  mouth  of  the  bony  vault.  This  condi- 
tion in  the  maxillary  bones  is  reached  between  the  seventh  and 
eighth  month  after  birth,  and,  almost  simultaneously  with  the 
completed  incasement  of  the  teeth  by  the  bone,  active  resorp- 
tion begins,  that  portion  of  the  bone  which  was  last  in  forming 
being  gradually  removed.  The  cause  of  the  resorption  of  the 
bone  may  readily  be  attributed  to  the  advancement  of  the 
tooth,  but  the  forces  which  are  responsible  for  this  latter  phe- 
nomenon do  not  appear  to  be  clearly  understood.  In  a  general 
way,  the  advancement  of  the  crown  may  be  said  to  result  from 
the  elongation  of  the  root  by  the  addition  of  dentin  to  its  free 
extremity.  But,  when  it  is  taken  into  consideration  that  the  cus- 
pid teeth,  both  deciduous  and  permanent,  have  their  roots  fully 
or  nearly  calcified  before  they  begin  to  advance  toward  the 
surface,  an  exception  to  the  generally  accepted  theory  is  estab- 
lished. The  eruptive  process  takes  place  first  in  the  anterior 
teeth,*  and  the  bone  overlying  the  labial  surface  is  first  removed. 

This  loss  of  the  bony  structure  is  continued  until  fully  one-half 

*  See  Description  of  the  Teeth  in  Detail,  p.  136. 


DEVELOPMENT   OF   THE   PERMANENT    TEETH.  335 

of  the  labial  surface  is  uncovered,  and,  as  the  crowns  continue 
to  advance  toward  the  surface,  they  assume  a  more  prominent 
position  in  the  arch,  and  thus  their  cutting-edges  become  bared. 
The  palatal  or  lingual  face  of  the  crypt  serves  a  double  purpose, 
forming  not  only  a  covering  to  the  deciduous  tooth,  but  also 
serving  the  permanent  tooth-sac  in  the  same  capacity.  This 
part  of  the  crypt  remains  unabsorbed,  the  tooth-crown  glides 
by  its  margin,  and,  after  penetrating  the  mucous  membrane, 
makes  its  appearance  in  the  mouth.  Closely  following  the 
resorptive  process  comes  a  rebuilding  of  the  parts,  until,  finally, 
when  the  tooth  is  fully  erupted,  it  is  firmly  supported  by  the 
new  bone  filling  in  about  the  base  of  the  root.  Accompanying 
the  eruption  of  the  anterior  teeth  and  their  establishment 
in  the  arch  is  an  increase  in  the  depth  of  this  portion  of  the 
jaw,  and,  as  the  molar  teeth  advance  and  assume  their  position, 
there  is  a  corresponding  increase  in  the  depth  of  the  jaw  in  this 
locality.  At  the  beginning  of  the  eruptive  period  the  roots  of 
the  deciduous  teeth  are  but  partially  calcified,  but  as  the  crowns 
advance  the  calcific  action  at  the  extremity  of  the  roots  is  con- 
tinued, and,  in  the  majority  of  instances,  by  the  time  the  crowns 
are  fully  erupted  the  roots  are  completely  formed.  During  the 
period  of  eruption  the  transitory  nature  of  the  alveolar  portion 
of  the  jaw-bone  is  made  manifest,  accommodating  itself  to  the 
growth  of  the  teeth  as  well  as  to  their  change  of  position.  The 
free  margins  of  the  alveolar  walls  are  taking  on  new  structure, 
which  advances  with,  and  becomes  adapted  to,  the  base  of  the 
tooth-root.  Coincident  with  this  the  deeper  portion  of  the 
alveolar  process  is  formed  by  a  rapid  fiUing-in  about  the  root 
as  the  tooth  travels  onward  to  assume  its  final  position  in  the 
jaw.  The  eruption  of  the  teeth  is  usually  by  pairs,  with  a  slight 
intermission  between  each  class.  The  central  incisors  first  make 
their  appearance,  followed  by  the  laterals,  after  which  the  first 
molars  are  erupted.  The  cuspids  usually  follow  the  first  molars, 
and,  finally,  the  second  molars  take  their  place  in  the  arch. 
While  this  brief  description  of  the  eruption  of  the  teeth  refers 
to  the  deciduous  organs  only,  the  process  in  the  permanent 
teeth  is  almost  identical  with  this.      Further  reference  to  the 


336  ANATOMY. 

eruption  of  the  permanent  teeth  will   be  made  in  connection 
with  the  degeneracy  of  the  temporary  set. 

To  return  to  the  subject  of  tooth-development,  attention  is 
called  to  figure  227,  prepared  from  a  dissection  upon  a  nine- 
months-old  child,  the  illustration  representing  the  hard  palate, 
or  roof  of  the  mouth,  at  this  period.  The  four  incisor  teeth 
have  made  their  appearance,  the  labial  surfaces  of  the  crowns 
being  fully  exposed,  while  those  facing  the  palate  are  but 
slightly  uncovered.  The  approach  of  the  remaining  deciduous 
teeth  is  plainly  indicated  by  the  fullness  of  the  alveolar  borders, 
and  the  margins  of  the  crypts  are  now  being  removed  by 
resorption. 


Fig.  227. — Hard  Palate  from  a  Nine-months-old  Child,  Actual  Size. 


If  the  mucous  membrane  should  be  removed,  the  crowns  of 
the  advancing  teeth  would  be  brought  to  view  after  the  removal 
of  the  walls  of  the  tooth-sacs,  while  the  approaching  cuspids  and 
second  molars  yet  remain  partly  covered  by  an  arching  over  of 
the  walls  of  the  crypts  ;  the  resorptive  process  has  also  begun 
in  these  parts.  It  would  also  be  observed  that,  while  the  walls 
of  the  crypts  are  molded  to  the  outlines  of  the  tooth-crowns, 
there  exists  a  well-defined  interspace  between  the  two.  During 
the  growth  of  the  tooth  this  interspace  is  filled  by  the  walls  of 
the  tooth-sacs,  and  even  after  the  teeth  have  passed  the  saccular 
stage  of  development,  and  assumed  their  positions  in  the  mouth, 


DEVELOPMENT   OF   THE   PERMANENT   TEETH.  337 

there  yet  remains  between  the  roots  and  the  alveolar  walls 
a  slight  space  which  is  occupied  by  the  alveolodental  mem- 
brane. 

The  next  dissection  is  one  upon  the  superior  maxillae  of  a 
two-year-old  child  (Fig.  228),  representing  the  roof  of  the 
mouth  of  this  subject.  In  this  specimen  it  will  be  noticed  that 
all  of  the  deciduous  teeth  are  erupted  with  the  exception  of  the 
second  molars.  The  lingual  surfaces  of  the  incisors  are  fully 
uncovered,  while  in  the  cuspids  the  labial  surfaces  are  much 
more  exposed  than  the  lingual.  In  figure  229  the  mucous 
membrane  and  sufficient  of  the   bone  have  been  removed  to 


Fig.  22S. — Hard  Palate  from  a  Two-year-old  Child,  Actual  Size. 


expose  the  tooth-sacs  of  the  developing  permanent  teeth.  The 
dissection  furnishes  no  additional  information  over  that  ob- 
tained from  figure  22S,  excepting  that  the  primitive  follicles  for 
the  permanent  second  molars  make  their  appearance  at  this  time. 
At  this  early  period  the  jaw  has  not  lengthened  sufficiently  to 
permit  of  these  follicles  occupying  their  future  position  ;  conse- 
quently they  are  found  generating  immediately  over  the  tooth- 
sacs  of  the  first  permanent  molars.  As  the  first  molars  advance 
and  the  jaw  lengthens  backward,  these  follicles  will  be  carried 
to  the  distal  by  the  extension  of  the  mucous  membrane,  to  which 
they  are  firmly  adherent.      If  these  follicles  were  to  be  dissected 


33S  ANATOMY. 

at  this  time,  the  papillae  would  be  without  definite  form,  showing 
that  the  early  function  of  the  enamel  organ  has  not  yet  begun. 
The  tooth-sacs  containing  the  permanent  lateral  incisors  are 
found  immediately  beneath  the  palatal  plates,  and  frequently 
during  their  earlier  life  they  are  not  even  protected  by  the 
bone,  being  in  immediate  contact  with  the  mucous  membrane. 
On  account  of  the  imperfect  protection  frequently  afforded  these 
sacs,  the  germs  are  sometimes  injured  and  the  teeth  fail  to  make 
their  appearance. 


Sac  for  Perma- 
nent Second 
Molar 


Fig.  22g. — Roof  of  the  Mouth  of  a  Two-year-old  Child. 


In  figure  230  the  walls  of  the  sacs  shown  in  figure  229  have 
been  opened,  and  the  relations  existing  between  the  first  and 
second  dentition  at  the  end  of  the  second  year  become  apparent. 
The  crowns  of  the  permanent  incisors  are  deeply  set  in  the 
substance  of  the  jaw,  while  the  partially  calcified  crowns  of  the 
permanent  laterals  are  in  close  proximity  to  the  palatal  surface. 
The  partially  formed  crowns  of  the  permanent  cuspids  are  still 
more  deeply  seated  in  the  substance  of  the  jaw  than  those  of 
the  central  incisors,  and  are  not  visible  in  the  illustration.  In 
this  connection  it  will  be  well  to  again  refer  to  the  gubernaculum, 
and  to  its  function — that  of  directing  the  tooth  to  its  proper 
position  in  the  arch.  By  reference  to  the  illustration  the  crowns 
of  the  permanent  teeth  will  be  observed  heading  in  various 
directions,   and,   while  in   this  instance  there    appears   to  be  a 


DEVELOPMENT   OF   THE   PERMANENT   TEETH.  339 

general  tendency  for  them  to  advance  and  assume  their  proper 
positions  in  the  arch,  in  many  cases  they  will  be  found  directed 
at  right  angles  to  the  point  at  which  they  should  emerge  from 


Fig.  230. — Development  of  the  Teeth  about  the  Second  Ye.4r. 

the  bone.     The  gubernaculum,  which  appears  to  be   nothing 
other  than  an  elongation  of  the  follicular  walls,  not  only  directs 


Tootli-sac  for  First  Bicuspid 
Fig.  231. — Dissection  of  Lingual  Face  of  Lower  Jaw,  Child  Nine  Months  Old. 

the  tooth  by  the  tension  of  its  fibers,  but  the  foramen  which 
its  presence  creates  stimulates  the  resorptive  action  over  the 
tract  to  be  traveled  by  the  tooth.      Figure   231    was  prepared 


ANATOMY. 


for  the  purpose  of  better  showing  the  gubernacula,  and  the 
manner  of  connecting  the  tooth-sacs  with  the  oral  mucous 
membrane.  This  condition  is  well  shown  in  the  anterior  teeth, 
the  tooth-crowns  having  receded  well  toward  the  body  of  the 

jaw.  The  follicle  for  the  first 
bicuspid  may  be  observed  at- 
tached to  the  lingual  face  of 
the  deciduous  molar  sac,  and 
the  leading  cord  is  not  yet  an 
adjunct  to  the  developmental 
process,  but  this  structure  will 
make  its  appearance  as  the  fol- 
licle recedes  from  the  surface. 

In  figure  232  the  deciduous 
molars  have  been  removed 
from  the  jaw  and  the  relations  existing  between  these  teeth 
and  the  developing  bicuspids  is  shown.  The  tooth-follicles 
for  the  succedaneous  teeth  are  found  immediately  beneath  the 
gingival  margin,  and  apparendy  attached  to  the  deep  layer  of 
the   mucous    membrane.     This    relationship  between   the    per- 


FiG.  232. — Deciduous  Molars  with 
Tooth-sacs  for  Bicuspids  Attached 
TO  THE  Gingival  Tissue. 


Deciduous  Incisors 


Fig.  233. — Same  as   Figure  231,  with   Tooth-sacs  Opened  Showing  Developing 
Teeth  in  the  Jaw. 


manent  and  temporary  organs  is  present  about  the  eruptive 
period,  but  as  the  deciduous  tooth  advances  and  the  permanent 
tooth-sac  recedes,  the  two  organs  become  more  widely  sepa- 
rated, and  the  permanent  follicle  is  connected  to  the  surface 


DEVELOPMENT   OF   THE   PERMANENT   TEETH.  341 

only  by  the  elongated  follicular  fibers  which   form   the  guber- 
naculum. 

Decalcification  of  the  Deciduous  Teeth. — By  the  close 
of  the  second  year  the  twenty  deciduous  teeth  have  taken  their 
place  in  the  dental  arch,  their  roots  have  become  fully  calcified, 
and    the  apical    foramina   established ;    it   is  only  for  a  short 


Fu;.  234. — Development  of  the  Teeth  about  the  Sixth  Year. 


period,  however,  that  they  remain  thus  perfect,  the  process  of 
decalcification  beginning  about  the  fourth  year.  This  resorptive 
action  begins  at  the  apical  extremities  of  the  roots  and  gradu- 
ally progresses  in  the  direction  of  the  crowns.  Commencing 
about  the  fourth  year  with  the  central  incisor,  decalcification 
takes  place  in  the  teeth  in  the  order  of  their  eruption,  the 
lateral  incisor  following  the  central,  the  first  molar  followino-  the 


342  ANATOMY. 

lateral,  etc.  By  reference  to  figure  234  an  approximate  idea 
of  tiie  progress  of  decalcification  may  be  obtained,  and  it  will 
be  observed  that  about  three  years  elapse  from  the  beginning 
of  this  rather  obscure  process  to  its  completion,  and  the  final 
casting-off  or  shedding  of  the  tooth-crowns.  In  reference  to  the 
causation  of  this  dissolution  of  the  deciduous  teeth,  but  little 
appears  to  be  known.  It  has  been  said  to  result  from  the 
presence  and  pressure  of  the  advancing  permanent  teeth,  but 
there  is  no  question  but  that  it  occurs  absolutely  indepen- 
dent of  these  organs,  decalcification  frequently  taking  place 
when  from  some  obscure  reason,  one  or  more  of  the  successional 
teeth  are  absent.  During  the  entire  period  of  root  decalcifica- 
tion, the  pulp  of  the  tooth,  which  is  also  involved  in  the  destrucv 
tion,  retains  its  vitality,  but  with  the  loss  of  vitality  in  the  pulp 
resorption  of  the  root  ceases  ;  so  that  the  gradual  removal  of 
the  root-substance  must  be  considered  as  a  purely  physiologic 
action. 

Figure  234  shows  a  dissection  upon  the  jaws  of  a  six-year- 
old  child,  by  a  careful  study  of  which,  a  fair  knowledge  of  the 
extent  of  resorption  in  the  deciduous  teeth  at  this  period  may 
be  obtained. 

Advance  of  the  Permanent  Teeth. — By  referring  to  figure 
234,  the  relations  existing  between  the  deciduous  and  the  per- 
manent teeth  at  about  the  sixth  year  may  be  noted.  While 
the  crowns  of  the  deciduous  teeth  remain  in  position,  a  part  of 
the  space  formerly  occupied  by  their  roots  is  taken  up  by  the 
advancing  crowns  of  the  permanent  teeth,  the  latter  being 
calcified  but  little  beyond  their  cervical  lines.  Between  the 
seventh  and  eighth  years  the  crowns  of  the  deciduous  incisors 
are  cast  off,  and  gradually  the  crowns  of  the  permanent  incisors 
force  their  way  through  the  gum,  the  arch  by  this  time  having 
sufficiently  increased  in  size  to  accommodate  the  additional 
width  possessed  by  them.  Previous  to  this  time,  or  about  the 
sixth  year,  by  a  backward  extension  of  the  jaws,  the  perma- 
nent first  molars  have  erupted,  assuming  a  position  in  the 
arch  immediately  posterior  to  the  deciduous  second  molars. 
Between  the  tenth  and  eleventh  years  the  crowns  of  the  decidu- 
ous molars  are  lost,  and   the  bicuspids  advance  to   take   their 


DEVELOPMENT   OF  THE   PERMANENT   TEETH.  343 

places.  Usually  by  the  twelfth  year  there  has  been  sufficient 
increase  in  the  length  of  the  jaws  to  permit  of  an  additional 
tooth,  and  the  permanent  second  molar  gradually  takes  its 
position  immediately  posterior  to  the  first.  Between  the  twelfth 
and  thirteenth  year  the  deciduous  cuspids  are  lost  by  decalcifi- 
cation of  their  roots,  and  they  are  succeeded  by  the  permanent 
cuspids.     We    therefore    find,  by  the    fifteenth    year,  fourteen 


Fig.  235. — The  Completed  Dentition. 


fully  developed  teeth  occupying  the  dental  arch  of  each  jaw, 
the  full  number,  thirty-two,  or  sixteen  in  each  jaw,  not  being 
present  until  the  eruption  of  the  third  molar,  which,  like  the 
other  teeth  of  this  class,  is  compelled  to  await  accommodations 
by  a  further  increase  in  the  length  of  the  maxillary  bones. 
This  tooth  usually  takes  its  place  between  the  eighteenth  and 
twenty-first  years,  and  thus  completes  the  dentition  (Fig.  235). 


PART  II.— HISTOLOGY. 


CHAPTER   I. 

GENERAL  HISTOLOGY:  THE  TISSUES  OF  THE  BODY:  EPITHELIAL 
TISSUES;  CONNECTIVE  TISSUES;  MUSCULAR  TISSUES;  NERVOUS 
TISSUES. 

Part  first  of  this  work  has  been  devoted  to  a  gross  description 
of  the  mouth,  its  structures,  the  macroscopic  arrangement  of  the 
tissues  composing  the  various  parts,  their  relations  to  one  another 
as  made  manifest  by  dissection,  etc.  The  remaining  chapters 
will  be  given  up  to  the  study  of  the  intimate  structure  of  the 
oral  tissues  according  to  their  form  and  organization.  The 
necessity  and  value  of  thus  continuing  the  subject  may  be 
presented  as  follows  :  If  any  of  the  structures  or  organs  of 
the  mouth — such  as  the  mucous  membrane,  the  mucous  glands, 
or  the  muscular  fibers  common  to  various  parts — are  taken  and 
submitted  to  a  minute  dissection,  it  will  be  found  that  the 
component  parts  may  be  separated  into  smaller  and  smaller 
portions.  Take,  for  example,  the  muscles  of  the  tongue  :  these, 
to  the  unaided  eye,  are  distinctly  fibrous  in  appearance,  and 
these  fibers  may  be  readily  separated  from  one  another  by 
removal  of  the  interposed  substance.  In  like  manner  the  oral 
mucous  membrane  is  composed  of  a  dense  fibrous  connective 
tissue,  which  may  be  separated  and  examined  in  a  similar  way. 
But  the  possibilities  of  investigation  by  this  method  are  limited, 
as  the  parts  thus  dissected  are  similar  to  one  another,  and  are 
soon  reduced  to  microscopic  proportions,  and  when  thus  exam- 
ined certain  definite  anatomic  forms  present  themselves  which 
are  no  longer  capable  of  being  divided.  The  reduction  of  any 
structure  to  such  definite  proportions  without  artificial  mutila- 
tion or  disarrangement  of  its  parts  is  productive  of  what  is 
termed  an  anatomic  element.  It  is  only  in  very  rare  instances 
that  a  tissue  is  composed  of  a  single  anatomic  element,  but 
usually  two  or  more  kinds  are  mingled  together,  and  the  tissue 
thus  formed  derives  its  texture,  quality,  and  appearance  from 

344 


THE  EPITHELIAL  TISSUES.  345 

the  variety  and  number  of  anatomic  elements  present.  Again, 
taking  the  muscular  tissues  as  an  example,  they  are  found  to 
be  composed  principally  of  one  anatomic  element — muscular 
fiber,  arranged  in  parallel  bundles.  In  close  relation  with  the 
fibers  are  numerous  nerve-filaments  and  capillary  vessels,  the 
whole  being  surrounded  by  a  thin  layer  of  connective  tissue. 
Numerous  bundles  of  this  nature  unite  to  form  larger  bundles, 
with  larger  nerve-filaments  and  blood-vessels.  In  the  hard  struc- 
tures the  same  conditions  are  present,  so  that  a  definite  under- 
standing may  be  obtained  by  a  minute  examination  only. 

Beyond  the  tissues  and  the  anatomic  elements  is  the  elemen- 
tary organism — the  cell. 


Embryonal 

Mucous 
Membrane 


Fig.  236. — Vertical  Tr.^nsverse  .-MXiiu.N  ihrijuuh   Head  of  Human  Embryo,  about 
THE  Sixth  Week,  showing  Single  Buccal  Cavity.     X  3°- 

A  cell  is  a  structural  element,  primarily  oval  or  spheroid  in 
form,  capable  of  self-nourishment,  development,  and  reproduc- 
tion. It  is  composed  of  two  essential  parts — the  cell-substance, 
consisting  principally  of  a  soft  albuminous  matter,  the  proto- 
plas7n,  and  generally  a  well-defined  central  portion,  the  necessary 
part  of  a  typical  cell — the  p-iiclens.  The  nucleus  is  usually  round 
and  situated  near  the  center  of  the  cell-substance.  In  many 
instances  there  is  within   the   nucleus  a  small   spheric  body,  the 


346  HISTOLOGY. 

nucleolus,  which,  although  not  having  its  function  definitely 
established,  undoubtedly  is  subordinate  to  the  life  activity  of  the 
cell.  At  the  beginning  of  development  the  cells  composing  a 
part  are  all  of  the  same  general  form,  and  similar  in  other 
respects  ;  as  development  proceeds,  the  cells  arrange  themselves 
in  germ-layers,  and  coincident  to  this  change  in  relationship  cease 
to  resemble  one  another  and  become  differentiated.  By  this 
aggregation  of  similar  cells  tissues  are  formed  ;  and  by  a  com- 
bination of  different  tissues,  a  structure  of  definite  form  and 
function,  an  organ,  is  generated. 

The  tissues  of  the  body  are  divided  into  four  grand  divisions 
— namely,  epitlielial  tissue,  connective  tissue,  muscular  tissue,  and 
nervous  tissue.  During  early  life  these  tissues  are  composed 
of  similar  elements — of  cells  ;  but  as  growth  proceeds,  a  twofold 
change  takes  place,  in  one  of  which  the  cells  produce  a  special 
deposit,  which  accumulates  between  them  and  forms  the  inter- 
cellular substance ;  in  the  other  the  tissues  of  one  kind  become 
mixed  with  those  of  another. 

The  Epithelial  Tissues. — The  cells  composing  the  epithelial 
tissues  are  known  as  epithelial  cells  ;  they  are  definite  in  outline, 
and  consist  of  the  cell-substance,  protoplasm,  and  a  nucleus.  Two 
forms  are  most  frequently  met  with — the  flattened  or  squamous, 
and  the  columnar  or  prismatic.  The  sqtiavious  or  pavement  cells 
are  flattened  and  scaly;  in  the  former  the  nucleus  is  round,  while 
in  the  latter  it  is  flattened.  The  columnar  cells  present  a  greater 
variety  in  outline,  being  conic,  club-shaped,  or  spindle-shaped, 
sometimes  short  and  sometimes  long.  The  nucleus  is  inclined 
to  the  form  of  the  cell-substance,  usually  being  more  or  less 
oval  or  oblong.  In  size  the  epithelial  cells  are  quite  variable, 
this  difference  occurring  in  cells  of  the  same  part  as  well  as  in 
those  of  distinct  parts.  By  the  arrangement  of  the  cells  the 
character  of  the  epithelium  is  established  ;  thus,  if  they  are  dis- 
posed in  single  layer,  a  single-layered-  epithelium  is  produced  ; 
or  if  stratified,  forming  several  superposed  layers,  a  stratified 
epithelium  results.  The  consistency  of  the  epithelial  cells  is 
such  that  they  readily  succumb  to  the  pressure  of  neighboring 
cells,  resulting  in  much  variation  in  their  outline.  They  are 
cemented  to  one  another  by  an   exceedingly  thin  layer  of   an 


THE   CONNECTIVE   TISSUES.  347 

albuminous  cement-substance,  being  in  contact  by  their  flattened 
sides  or  by  irregularly  formed  processes,  the  result  of  force 
applied  by  adjacent  cells. 

A  third  class  of  epithelial  cells  are  those  columnar  cells  which 
are  beset  upon  their  free  surfaces  with  numerous  minute  pro- 
cesses which  during  life  are  constantly  vibrating;  these  are 
known  as  ciliated  cells.  Continuous  masses  of  epithelial  cells, 
"  epithelia,"  are  found  covering  the  surface  of  the  skin,  and  lining 
internal  canals  and  organs.  They  are  found  in  abundance, 
covering  all  parts  of  the  mucous  membrane  of  the  mouth,  and 
lining  the  numerous  ducts  which  empty  into  that  cavity.  In 
some  instances  the  epithelia  are  composed  of  a  single  stratum, 
in  others  of  a  number  of  strata,  and  the  following  varieties 
are  to  be  observed  :  simple  squamoits,  simple  columnar,  and 
simple  ciliated  epithelium,  when  a  single  stratum  is  present ; 
and  stratified  squamous,  stratified  columnar,  and  stratified  cili- 
ated epithelium,  when  the  several  strata  are  present.  The  epi- 
thelium of  the  mouth  is  principally  of  the  stratified  squamous 
variety.  Another  classification  is  that  known  as  glandular 
epithelium,  composing  the  numerous  glands  and  ducts  (see 
Glands  of  the  Mouth).  In  addition  to  this,  one  other  variety 
exists,  known  as  neuro-epithelium. 

One  of  the  special  characteristics  of  epithelial  tissue  is  the 
relatively  small  amount  of  intercellular  substance  as  compared 
to  the  cells  themselves. 

The  Connective  Tissues, — In  general  this  term  is  applied 
to  all  those  tissues  which  support  and  connect  other  tissues. 
While  the  cells  are  numerous,  they  are  proportionately  less 
numerous  than  in  the  epithelial  tissues.  In  the  connective  tissues 
the  intercellular  substance  predominates  where  it  is  prominently 
developed  and  variously  differentiated.  The  intercellular  sub- 
stance is  characteristic  of  the  connective-tissue  group,  and  is  espe- 
cially interested  in  its  function.  By  a  variation  in  the  quantity, 
nature,  and  disposition  of  the  intercellular  substance,  three  classi- 
fications are  formed — namely,  fibrous  connective  tissue,  cartilage, 
and  bone  (including  dentin  and  cementum).  Each  of  these 
groups  is  subdivided  into  several  varieties,  but  in  all  instances 
the  intercellular  substance  is  to  be  distinguished  from  the  cells. 


348  HISTOLOGY. 

Fibrous  Connective  Tissue. — This  tissue  is  present  in  the  skin 
and  mucous  membrane,  in  the  intermuscular  tissues,  in  tendons, 
in  fascia  and  aponeuroses,  and  in  the  tissues  connecting  various 
organs.  It  is  composed  of  a  meshwork  of  fine  fibers  of  two 
kinds.  The  first,  which  makes  up  the  greater  part  of  the  tissue, 
is  formed  of  very  fine,  white,  structureless  fibers  arranged  closely 
in  bundles  and  bands  crossing  and  intersecting  in  all  directions. 
The  second  variety,  or  the  yellow,  elastic  fiber,  has  a  much 
sharper  and  darker  outline,  not  arranged  in  bundles,  but  is  inti- 
mately mingled  with  the  white  fibers  by  twisting  around  and 
among  its  filaments.  These  are  known  as  the  elementary  con- 
nective-tissue fibers.  The  size  of  the  connective-tissue  bundles 
depends  upon  the  number  of  elementary  fibers  present,  and  by 
a  variation  in  the  arrangement  of  the  bundles  variety  in  the 
character  of  the  fibroconnective  tissue  is  produced  in  different 
localities.  When  the  fibrous  connective  tissue  is  formed  into  an 
unbroken  mass,  as  in  mucous  membrane,  the  minute  bundles  are 
collected  into  smaller  or  larger  groups  (the  trabeculcB),  and  these 
are  in  turn  associated  into  groups.  In  the  skin  and  mucous 
and  serous  membranes,  the  trabeculae  of  the  connective-tissue 
bundles  are  separated,  and,  by  crossing  and  recrossing  one 
another,  form  a  dense,  fan-like  structure.  In  other  tissues,  as  the 
tendons  and  fascia,  the  bundles  are  arranged  in  parallel  layers. 
In  the  submucous  tissues  the  connective-tissue  fibers  are 
loosely  woven,  the  fibers  crossing  and  intermingling,  with  the 
intervening  spaces  unusually  large,  resulting  in  a  loose,  flabby 
tissue — areolar  tissue.  Three  varieties  of  fibrous  connective  tis- 
sue are  distinguished — namely,  mucous  connective  tissue,  fibril- 
lar connective  tissue,  and  reticular  connective  tissue.  Mucous 
connective  tissue  consists  of  an  abundance  of  undifferentiated 
intercellular  substance  in  which  are  a  few  bundles  of  fine  fibrils 
and  a  number  of  round  or  oval  cells.  Fibrillar  connective  tissue, 
or  areolar  tissue,  consists  of  an  abundance  of  intercellular  sub- 
stance differentiated  into  connective-tissue  fibers  and  cells.  Retic- 
ular connective  tissue,  as  its  name  implies,  is  a  network  of  slender 
bundles  of  connective  tissue  associated  with  flattened,  nucleated 
cells. 

The   fibrous    connective-tissue    cells   are   few   in    number,  of 


CARTILAGE.  349 

several  varieties,  and  variously  shaped,  being  flattened,  stellate, 
or  apparently  distorted  by  pressure  from  surrounding  cells  or 
fibrous  bundles.  In  the  mucous  membrane  the  cells  are  oblong 
and  somewhat  flattened,  having  many  branches  which  reach 
out  and,  uniting  with  like  processes  from  neighboring  cells,  form 
a  network.  Other  connective-tissue  cells  are  comparatively 
larger,  oval  or  rounded  in  form,  granular  in  appearance,  rich  in 
protoplasm,  and  are  known  as  plasma-cells.  The  body  of  con- 
nective-tissue cells,  besides  containing  a  nucleus,  frequently 
contains  pigment-granules  ;  these  are  known  as  pigment-cells. 
These  are  seldom  found  in  mucous  or  serous  membranes, 
being  principally  confined  to  the  integument.  Fat-globules 
may  also  be  found  in  fibrous  connective  tissue,  and  when  of 
considerable  size  unite  and  form  a  rounded  cell,  called  a  fat-cell. 
Numerous  fat-cells  uniting,  and  well  supplied  with  blood-vessels 
and  nerves,  form  adipose  tissue,  or  fat.  Fat-cells  are  frequently- 
found  in  areolar  tissue  as  well.  When  fibrous  connective  tissue 
is  immediately  contiguous  to  epithelium,  it  becomes  somewhat 
modified  and  a  new  membrane  is  formed,  called  the  basement 
membrane,  or  menibrana  propria.  This  membrane  is  a  thin, 
transparent,  structureless  layer,  and,  when  in  connection  with 
those  mucous  membranes  provided  with  a  layer  of  vascular 
fibrocellular  tissue,  may  appear  as  the  formative  substance  out 
of  which  successive  layers  of  epithelial  cells  are  generated.  In 
the  ducts  and  glands — for  example,  the  salivary  glands — the 
basement  membrane  forms  the  proper  walls  of  the  tubes,  and 
the  cells  here  generated,  and  corresponding  to  the  epithelial 
cells  of  the  coarser  mucous  membranes,  are  known  as  gland- 
cells,  rather  than  epithelial  cells.  This,  however,  is  a  distincdon 
without  a  perceptible  difference,  the  location  and  function  as 
secreting  cells  being  alike  in  each. 

Cartilage. — Cartilage  is  a  semi-opaque,  non-vascular  tissue, 
white  in  color,  and  composed  of  a  matrix  containing  nucle- 
ated cells.  The  matrix  is  somewhat  elastic  and  rather  dense. 
The  cells  are  simple  in  form,  being  spheric  or  slightly  inclined 
to  angularity.  The  variation  in  the  character  of  cartilage  is 
due  rather  to  the  difference  in  the  character  of  the  matrix 
than   to   the   cellular  structure,   the    principal    variation    in    the 


350  HISTOLOGY. 

cells  being  in  their  size.  The  cells  lie  in  the  spaces  or  la- 
cunae of  the  matrix,  which  they  completely  fill.  Investing  the 
free  surface  of  most  cartilaginous  tissue  (articular  cartilage  ex- 
cepted) is  a  thin  but  tough  and  firm  fibrous  membrane — xX^o.  peri- 
chondrium. This  membrane  is  well  supplied  with  blood-vessels 
and  nerves,  and  is  essential  to  the  growth  and  maintenance  ot 
the  cartilage.  There  are  three  varieties  of  cartilage — namely, 
hyaline  cartilage,  elastic  cartilage,  and  fibrocartilage. 

Hyaline  cartilage  is  of  a  faint  pearly-blue  color,  slightly  trans- 
parent, and  is  found  investing  the  articular  ends  of  the  bones — 
for  example,  the  condyles  of  the  mandible  ;  also  forming  the  costal 
and  nasal  cartilages,  as  well  as  those  of  the  trachea,  bronchi,  and 
a  part  of  the  larynx.  Hyaline  cartilage  is  distinguished  by  a 
granular  or  homogeneous  matrix.  The  cells,  which  contain  a 
nucleus  with  nucleoli,  are  usually  grouped  together  in  patches, 
and  are  somewhat  irregular  in  outline,  appearing  flattened  near 
the  free  surface  of  the  tissue  in  which  they  are  placed,  and  in- 
clined to  be  perpendicular  to  the  surface  in  the  more  deeply- 
seated  portions.  The  matrix  is  dimly  granular  in  appearance, 
resembling  ground  glass,  and  receiving  its  name  from  this  fact. 
That  part  of  the  cartilage  close  to  the  perichondrium  is  sup- 
plied with  cells  much  smaller  than  those  occupying  the  lacunae 
In  the  substance  of  the  mass,  and  the  growth  of  the  cartilage  is 
most  active  in  this  part.  Lining  each  lacuna  is  a  delicate  mem- 
brane (the  capsule),  which  primarily  is  but  partly  filled  out,  but 
as  the  cell  or  cells  increase  in  size,  this  membrane  is  carried  to 
the  walls  of  the  lacuna.  Articular  hyaline  cartilage  is  non- 
vascular, being  nourished  by  the  blood-vessels  of  the  bone 
beneath. 

Elastic  cartilage  is  of  a  dull-yellow  color,  and  is  sometimes 
called  yellow  cartilage.  It  is  not  present  in  the  mouth,  but 
occurs  in  the  external  ear,  in  the  epiglottis,  and  in  part  of  the 
larynx.  Its  structural  composition  is  quite  similar  to  hyaline 
cartilage,  but  may  be  distinguished  from  it  by  a  network  of  fine 
elastic  fibers  which  penetrate  the  matrix.  The  cells  are  rounded 
or  oval,  containing  nuclei  and  nucleoli. 

Fibrocartilage  is  yellowish  or  milky  white  in  color,  and  is 
much  more  widely  distributed   throughout  the   body  than  the 


elastic  variety.  It  is  present  in  the  temporomandibular  articula- 
tion. Like  those  previously  described,  it  is  composed  of  cells 
and  a  matrix,  the  latter  being  made  up  of  fibrous  connective 
tissue  arranged  in  bundles,  and  for  this  reason  it  is  scarcely 
deserving  the  name  of  cartilage,  only  that  in  other  portions 
continuous  with  it  cartilage-cells  may  be  found  in  abundance. 
Between  the  strata  of  the  fibrous  bundles  are  numerous 
nucleated  cells,  which  are  oval  and  more  or  less  flattened,  and 
each  enveloped  in  a  delicate  capsule. 

Cartilage  is  further  classified  into  two  divisions, — temporary 
zx\.^  permanent, — the  former  term  being  applied  to  that  kind  of 
cartilage  which  in  the  fetus  and  in  youth  is  destined  to  be  con- 
verted into  bone  (for  example,  Meckel's  cartilage)  ;  the  latter 
class  including  all  those  cartilages  w^hich  are  generated  as  such, 
and  continue  to  serve  in  that  capacity.  Temporary  cartilage 
closely  resembles  the  hyaline  variety,  being  formed  of  a  matrix 
in  the  lacunae  of  which  the  cells  are  located.  These  cells,  how- 
ever, are  not  grouped  together  as  in  hyaline  cartilage,  but  are 
more  uniformly  distributed  throughout  the  matrix. 

Bone. — Bone  is  mainly  composed  of  tricalcium  phosphate 
and  cartilage.  The  matrix  of  osseous  tissue  has  a  distinguish- 
ing feature  produced  by  the  blending  of  organic  and  inorganic 
substances,  resulting  in  hardness,  solidity,  and  elasticity.  The 
combination  of  organic  and  inorganic  elements  in  bone  is  of 
such  a  nature  that  either  part  may  be  removed  without  destroy- 
ing the  other.  The  matri.x  is  composed  of  the  salts  of  lime, 
especially  calcium  phosphate,  and  of  slender  fibrils  united  by  a 
cement-substance  into  bundles  of  various  sizes.  The  cement- 
substance  is  chiefly  composed  of  insoluble  lime-salts,  princi- 
pally carbonates  and  phosphates.  These  two  kinds  of  struc- 
ture are  found  to  be  present  in  different  parts  of  the  same 
bone,  forming  a  dense  or  compact,  and  a  spongy  or  can- 
cellated tissue.  The  former  occur  in  the  shaft  of  long  bones 
and  in  the  outer  layer  of  flat  or  irregularly  formed  bones.  Can- 
cellated bone-substance  occurs  in  the  extremities  of  the  long 
bones  and  in  the  interior  of  flat  and  irregular  bones.  The 
irregularly  formed  maxillary  bones  give  place  to  both  kinds  oi 
bony  structure  ;  the  external  layer  of  the  superior  maxillae  and 


352 


HISTOLOGY. 


the  body  and  rami  of  the  inferior  maxilla  are  composed  of  com- 
pact tissue,  while  the  interior  of  these  bones  and  the  condyloid 
processes  of  the  mandible  are  spongy  or  cancellated  in  their 
nature.  When  examined  by  the  microscope  the  bony  substance 
is  found  occupied  by  numerous  little  spindle-shaped  spaces — 
lacuncB.  Branching-  out  from  these  in  various  directions  are 
minute  canals — canaliculi — which  anastomose  with  similar 
canals  from  neighboring  lacunse.  In  the  maxillary  bones  no 
other  canals  than  these  may  be  visible,  but    if   a    transverse 


Fig.  237. — Developing  Bone.     X  4°- 

section  be  cut  through  one  of  the  long    bones,   an  additional 
space  makes  its  appearance  (Fig.  238). 

These  spaces  are  known  as  the  Haversian  canals.  They  are 
circular  in  outline  and  appear  as  a  center  for  a  small,  circular 
district  mapped  out  by  concentric  layers,  the  lacunse  and  canal- 
iculi following  the  same  concentric  plan,  and  through  each  other 
communicating  with  the  Haversian  canals.  The  general  direc- 
tion of  the  Haversian  canals  is  longitudinal  with  the  long  axis 
of  the  long  bones,  and  in  the  flat  or  irregular-shaped  bones 
they  are  somewhat  irregular  in  formation  and  ramify  in  various 
directions.      In    the    osseous    matrix    each    lacuna    contains    a 


BONE.  353 

bone-cell.  These  are  nucleated,  protoplasmic  cells.  In  devel- 
oping bone,  these  cells,  which  do  not  completely  fill  the  lacunae, 
are  connected  by  numerous  branches  or  processes  passing 
through  the  canaliculi;  in  older  bone  very  few  processes  are 
observed. 

There  are  two  processes  by  which  bone  may  be  prepared 
for  histological  examination,  by  one  method  which  results  in 
the  destruction  of  the  organic  elements,  or  by  another  which 
removes  the  inorganic  elements.  In  the  former  process  the 
organic  matter  is  removed  by  simply  drying  the  structures,  after 


Concentric  ^  ''^^  -  ^  j,  \^  "' 

Haversian      ^f^^.       '      .     -■  ,    ^^  \ 


r''.    ',"■    ,  ^  '■---  *^.--^ 


Fig.  23S.— Transverse  Section  through  Shaft  of  Long  Bone.     X  3°- 

which  thin  sections  may  be  prepared  and  carefully  examined 
under  the  microscope,  when  the  Haversian  canals,  lacunae,  and 
canaliculi  will  be  seen  forming  a  complete  concentric  network. 
In  the  latter  method  the  inorganic  substance  is  removed  by 
immersing  a  fresh  bone  in  dilute  picric  acid,  C6H3(NOJ30H, 
which  readily  decalcifies  it,  and  when  properly  prepared  sections 
are  placed  under  the  microscope  the  organic  contents  of  the 
lacunae  and  canaliculi  alone  are  visible. 

The  concentric  laminee  of  bone  is  riveted  together  by  numer- 


354  HISTOLOGY. 

ous  delicate  rods  or  processes  named  Sharpeys  fibers,  these 
delicate  fibers  passing  through  the  laminae  to  perform  this  office. 
Periosteum  and  Bone-marrow. — The  interstices  of  spongy 
bone  are  filled  with  a  soft  mass, — the  bone-marrow, — and  the 
external  surface  of  the  bone  is  covered  by  a  fibrous  membrane 
— the  periosteum.  This  membrane  is  absent  where  bones  are 
joined  to  each  other  by  ligament  or  cartilage,  and  over  articular 
surfaces.  The  periosteum  is  a  compact  connective-tissue  mem- 
brane.     It  consists  of  two  layers:  an  obiter, 'vibxow's,  layer  rich  in 


Fig.  239. — Longitudinal  Section  of  Long  Bone.     X  30- 


blood-vessels,  which  forms  the  connection  with  adjacent  struc- 
tures ;  an  inner  or  osteogenetic  layer  containing  few  blood- 
vessels, loose  in  texture,  but  rich  in  elastic  fibers  and  spheric 
connective-tissue  cells,  with  oval  nuclei.  These  are  the  forma- 
tive cells  of  bone  and  are  called  osteoblasts.  These  cells  appear 
in  the  lower  strata  of  the  inner  layer,  or  the  layer  in  contact 
with  the  bone,  and  are  especially  numerous  during  the  period 
of  development.  Through  the  blood-vessels  of  the  bone  the 
marrow,  internally,  is  placed  in  communication  with  the  perios- 
teum externally  ;   small  branches  given  off  from   the  numerous 


NON-STRIATED    MUSCULAR   TISSUE,  355 

arteries  and  veins  of  the  periosteum  enter  the  Haversian  canals, 
upon  which  they  pass  to  the  canaliculi,  thus  communicating  with 
the  blood-vessels  of  the  marrow.  In  like  manner  numerous 
nerves  enter  the  substance  of  the  bone,  first  passing  into  the 
Haversian  canals,  after  which  they  become  closely  associated 
with  the  minute  blood-vessels  and  are  distributed  to  the  perios- 
teum and  bone-marrow.  The  bone-marrow,  besides  filling  the 
interstices  of  the  spongy  substance,  is  also  found  occupying  the 
central  cavity  of  long  bones,  and  in  the  larger  Haversian  canals. 
The  marrow  is  of  two  varieties,  distinguished  by  its  color,  being 
either  red  or  yellow.  Red  marrow  is  found  in  the  flat  bones 
(including  the  maxillae),  the  vertebrae,  and  ribs,  while  yellow 
marrow  occurs  in  the  long  bones  of  the  extremities.  Red  mar- 
row is  composed  of  a  delicate  connective-tissue  network  sup- 
porting, besides  the  marrow-cells,  a  few  fat-cells  and  giant-cells. 
In  the  long  bones  the  yellow  marrow  is  surrounded  by  a  con- 
nective-tissue membrane  lining  the  medullary  canals.  Marrow- 
cells  and  giant-cells  are  present  in  abundance.  Marrow  is  very 
vascular  and  contains  many  osteoblasts. 

Dentin. — This  structure,  as  well  as  cementum,  which  in  many 
particulars  closely  resembles  bone,  will  be  fully  considered  in 
connection  with  the  histology  of  the  tissues  of  the  teeth. 

Muscular  Tissues. — The  muscle-fibers,  or  the  structural 
elements  of  the  muscular  tissues,  are  divided,  according  to  the 
arrangement  of  their  bundles,  into  two  classes — non-striated 
and  striated. 

Non-Striated,  Smooth,  or  Involuntary  Muscular  Tissue. 
— This  tissue  consists  of  contractile  fiber-cells,  which  are  elon- 
gated, spindle-shaped,  and  cylindric,  with  exceedingly  elongated 
extremities,  which  become  shorter  and  thicker  through  contrac- 
tion. They  are  quite  variable  in  length  (yV  to  ^i^j  of  an  inch), 
and  are  composed  of  a  pale,  homogeneous-looking  protoplasm, 
each  inclosing  an  elongated  or  rod-shaped  nucleus,  which  is 
flattened  if  the  cell  is  so  formed.  The  muscular  fibers  are 
firmly  bound  together  by  a  cement-substance,  forming  fasciculi, 
which  in  turn  are  collected  into  strata  or  membranes,  which 
may  be  disposed  parallel,  or  crossing  and  recrossing,  forming 
an  intricate  network.     The  connective-tissue    septa  provide  a 


356 


HISTOLOGY. 


passageway  for  the  larger  blood-vessels,  while  the  capillaries 
penetrate  the  fasciculi  forming  a  complicated  network  with 
oblong  meshes.  Involuntary  muscular  tissue  is  not  found  in  the 
mouth  except  in  the  ducts  of  the  salivary  glands. 

Striated  or  Voluntary  Muscular  Tissue. — Striated  mus- 
cular tissue  is  composed  of  long,  cylindric  fibers,  which  are 
regularly  transversely  striated.  In  most  instances  their  extrem- 
ities are  attached  to  bones  by  means  of  tendons,  as,  for  example, 
the  cheek-  and  lip-muscles.  The  fibers  are  grouped  together  by 
fibrous  connective  tissue   into  various   sized    bundles,  forming 


Fig.  240. — Transverse  Section  of  Striated  or  Voluntary  Muscular  Tissue.     X  A°- 


fasciculi.  There  is  much  variation  in  the  length  of  the  fibers 
composing  the  fasciculi  in  different  muscles.  In  most  instances 
the  fasciculi  which  serve  to  make  up  the  bundles  of  a  single  mus- 
cle condnue  parallel  with  one  another  throughout  their  length. 
Surrounding  the  muscular  fasciculi,  and  forming  a  covering  or 
sheath  for  the  individual  bundles,  is  a  layer  of  connective  tissue 
called  the  perimysium,  and  passing  from  this  into  the  substance 
of  the  bundle  is  a  delicate  connective  tissue,  the  endomysium, 
which  separates  the  individual  fibers  from  one  another.     The 


STRIATED   OK   INVOLUNTARY   MUSCULAR    TISSUE.  357 

former  structure   carries  the  larger  blood-vessels   and   nerve- 
fibers,  while  the  latter  supports  the  capillaries. 

Each  muscular  bundle  may  again  be  divided  into  smaller 
bundles,  which  in  turn  are  ensheathed  iri  a  similar  manner  and 
further  divisible,  so  continuing  until  the  primitive  fasciculi,  or 
so-called  muscular  fiber,  is  reached.  Striped  muscular  fiber 
consists  of  a  structureless,  elastic  sheath,  the  sarcoleinvia,  which 
structure  represents  the  cell-membrane,  and  closely  invests  a 
number  of  filaments  or  fibrils.  Besides  the  fibrillee,  there  is  con- 
tained within  this  fine,  structureless,  transparent  membrane  the 


Fig.  241. — Striated  or  Voluntary  Muscular  Tissue.     X  40- 


sarcoplasm,  a  faintly  granular  substance  resembling  protoplasm, 
but  not  identical  with  it.  This  substance  serves  in  the  capacity 
of  a  matrix  for  the  fibrillae.  The  fibrillse  are  arranged  parallel 
to  one  another,  being  supported  by  the  sarcoplasm.  It  will 
thus  be  seen  that  each  fiber  of  a  striated  muscle  comprises  the 
sarcolemma,  the  muscle-nuclei,  the  fibrillae,  and,  finally,  the 
sarcoplasm,  filling  all  the  interstices,  first  between  the  fibrillae 
of  each  muscle-column,  between  the  columns  of  each  group, 
and  between  the  groups  themselves.     The  disposition  of  the 


358 


HISTOLOGY. 


sarcoplasm  may  be  most  favorably  studied  by  a  cross-section 

through  the  fibers,  appearing  as 
a  delicate  but  clear  network, 
within  the  meshes  of  which  are 
the  muscle-columns.  Striated 
muscular  fibers  are  usually  spin- 
dle-shaped, tapering  off  and  be- 
coming thinner  toward  their  ex- 
tremities. In  rare  instances 
they  are  branched  at  their  ends. 
This  condition  is  present  in  the 
tongue,  the  extremities  of  the 
fibers  passing  transversely  into 
the  oral  mucous  membrane, 
where  they  become  further  sub- 
divided. The  striated  or  vol- 
untary muscles  make  up  the 
muscular  tissues  of  the  lips, 
cheeks,  tongue,  and  soft  palate. 
Nervous  Tissues.  —  Until 
within  recent  times  it  has  been 
stated  that  the  nervous  tissue 
consists  of  two  histologic  ele- 
ments known  as  nerve-cell  and 
nerve-fiber;  that  these  two  ele- 
ments differed  not  only  in  their 
mode  of  origin,  but  in  their 
structure  and  physiologic  en- 
dowments. At  the  present  time 
it  is  believed  that  the  entire 
nervous  system  consists  of  an 
infinite  number  of  definite  in- 
dependent morphologic  units, 
which,  though  having  a  common 
origin  and  a  similarity  of  struc- 
ture, have,  nevertheless,  differ- 
ent functions  in  different  parts 
of  the  body.     This  neurologic 


Fig.  242. — Diagram  of  a  Neuron. — 
[Fi-om  Stok}-'s  "Histology.") 


NERVOUS   TISSUES.  359 

unit  has  been  termed  the  neuron,  and,  as  represented  schemati- 
cally in  figure  242,  may  be  said  to  consist  of:  First,  the  nerve- 
cell,  or  neurocyte  ;  second,  nerve-process,  or  axon  ;  third,  the 
end-tufts,  or  terminal  branches.  Each  of  these  three  main 
portions  of  the  neuron  presents  a  variety  of  secondary  features 
which  are  related  to  their  functional  activities. 

The  Nerve-cell,  or  Neurocyte. — The  nerve-cells  are  found  in 
the  cortex  of  the  brain,  in  the  interior  of  the  spinal  cord,  in  the 
various  ganglia  of  the  cerebrospinal  and  sympathetic  nervous 
systems,  and  in  the  organs  of  special  sense.  All  neurocytes  are 
the  modified  descendants  of  independent  oval  or  pear-shaped 


Epiiieur  u 


Perineurium 
Endoneuriun 


Fig.  243. — Transverse  Section,  Bundles  of  Nerve-fibers,  Human   Median  Nerve. 

X30- 


cells  (the  neuroblasts),  originating  from  the  epithelial  cells 
which  form  the  medullary  tube.  The  neurocyte  is  at  first 
smooth,  devoid  of  processes,  and  endowed  with  ameboid  move- 
ment. In  the  course  of  development  the  cells  project  a  greater 
or  less  number  of  processes  and  assume  a  variety  of  shapes 
and  sizes,  in  accordance  with  variations  in  functions;  thus,  the 
cells  may  be  spheroid,  pyramidal,  spindle-shaped,  stellate,  etc. 
The  body  of  the  cell  consists  of  a  protoplasmic  basis,  more  or 
less  granular,  containing  a  well-defined  nucleus  and  nucleolus. 
A  centrosoma   has  also   been   found   in   the  nerve-cell   in  many 


36o 


HISTOLOGY. 


situations.  There  is  no  evidence,  liowever,  of  the  existence  of 
a  cell-membrane.  From  the  body  of  the  neurocyte  there  arises 
one  or  more  protoplasmic  processes,  which,  passing  outward  in 
various  directions,  divide  and  subdivide  into  a  greater  or  less 
number  of  branches,  which  are  collectively  known  as  dendrites 
or  dendrons.  The  ultimate  subdivisions  and  terminations  of  a 
dendrite,  though  forming  an  intricate  feltwork,  always  end 
free,  never  anastomosing  with  one  another.  Arising  from  the 
cell-body,  the  dendrites  resemble  in  appearance  and  structure 
the  cell-protoplasm,  or  cytoplasm.     In  the  cortex  of  the  cerebrum 


•^ 


.■J^i 


Fig.  244. — Portion  of  Transverse  Section  of  Human  Median  Nerve.     X  200. 


and  in  the  cortex  of  the  cerebellum  the  dendrites  are  character- 
ized by  short,  lateral  projections  known  as  lateral  buds  or  gem- 
mules,  which  impart  to  the  dendrite  a  feathery  appearance. 

The  Axon,  or  Ne^^ve-processi — The  axon  is  the  first  outgrowth 
of  the  protoplasm  of  the  neuroblast,  but  with  the  development 
of  the  neurocyte  it  becomes  so  differentiated  from  the  dendrites 
that  it  can  be  readily  distinguished  from  them.  It  usually  arises 
from  a  cone-shaped  projection  of  the  cell-body,  though  occa- 
sionally it  arises  from  a  dendrite  itself.  It  is  characterized  by  a 
short,  regular  outline  and  a  hyaline  appearance.     The  majority 


THE   END-TUFTS,    OR   ARBORIZATIONS.  361 

of  the  cells,  especially  in  the  mammalia,  possess  but  one  axon, 
though  in  the  developing  ganglion-cells  of  the  spinal  nerves  two 
distinct  axons  are  present.  In  their  subsequent  development 
the  two  axons  appear  to  blend  together  to  form  but  a  single 
axon,  which,  at  a  short  distance  from  the  cell,  again  divides  into 
two  branches,  which  pursue  opposite  directions,  one  passing 
directly  into  the  spinal  cord,  the  other  toward  the  periphery. 
The  axon  may  continue  as  an  individual  structure  for  an  indefi- 
nite distance,  varying  from  a  few  millimeters  to  100  cm.  In  the 
former  instance  the  axon,  at  a  distance  of  a  few  millimeters  from 
the  cell,  breaks  up  into  a  number  of  branches,  which  form  an  intri- 
cate feltwork  in  the  neighborhood  of  the  cell.  This  type  of  cell  is 
not  widely  distributed,  being  confined  largely  to  the  cerebellum. 
In  its  course  the  axon,  more  especially  in  the  central  nervous  sys- 
tem, gives  off  a  number  of  side-branches  or  collaterals,  which 
do  not  differ  from  the  axon  itself,  either  in  structure  or  appear- 
ance. The  axon  of  the  peripheral  nerves,  especially  the  spinal 
nerves,  are  devoid  of  collaterals  throughout  their  extent,  except, 
perhaps,  in  the  immediate  neighborhood  of  the  cell.  The  more 
or  less  elongated  axon  becomes  inclosed  at  a  short  distance 
from  the  cell  wnth  a  thick  layer  of  fatty  material,  forming  a 
medulla  or  myelin,  inclosed  by  a  delicate  cellular  sheath  (the 
neurilemma),  and  thus  constitutes  what  is  commonly  known  as 
a  medullated  nerve-fiber.  In  the  central  nervous  system  the 
neurilemma  is  frequently  wanting.  In  the  sympathetic  system 
the  myelin  is  wanting,  though  the  axon  is  inclosed  by  a  deli- 
cate sheath  resembline  the  neurilemma,  thus  constituting  a  non- 
medullated  nerve-fiber.  The  collateral  branches  are  provided 
with  similar  investments. 

The  End  Tufts,  or  Arborizations. — Each  axon,  as  it  ap- 
proaches its  final  termination,  breaks  up  into  a  number  of 
branches,  which  vary  in  complexity  and  appearance  in  different 
regions.  They  are  always  free  from  any  medullary  investment, 
and  appear  to  be  formed  by  the  splitting  of  the  axon  into  a 
number  of  fine  filaments,  which  remain  independent  of  one 
another.  In  peripheral  organs,  as  muscles,  glands,  and  blood- 
vessels, the  tufts  are  in  direct  organic  connection.  In  the 'central 
nervous  system  the  end-tufts  are  in  more  or  less  intimate  rela- 
tion with  the  dendrites  of  other  neurons. 


362  HISTOLOGY. 

Nerves. — Nerves  are  to  be  regarded,  therefore,  as  groups  of 
axons,  with  their  medullary  investments  connecting  the  periph- 
eral organ  with  the  central  nervous  system. 

The  nerves  are  arranged  in  two  great  systems — the  cerebro- 
spinal and  the  sympathetic.  In  the  cerebrospinal  nerves  the 
conducting  media — the  nerve-fibers — are  arranged  in  parallel  or 
interlacing  bundles,  and  these  are  further  grouped  into  nerve- 
branches  or  nerve-trunks.  The  bundles  are  connected  by  inter- 
vening fibrous  connective  tissue  (the  epinezirhtiii),  and  through 
this  tissue  the  principal  blood-vessels  ramify  to  supply  the  nerve- 
trunks,  together  with  a  plexus  of  lymphatics  and  numerous  fat- 
cells  and  plasma-cells. 

The  size  of  the  nerve-bundles,  or  funiadi,  is  regulated  accord- 
ing- to  the  size  and  number  of  nerve-fibers  which  they  contain. 
Investing  each  funiculus,  or  primary  bundle  of  nerve-fibers,  is 
a  connective-tissue  sheath — the  perineurmm.  The  fibers  com- 
posing this  sheath  are  arranged  in  lamellae,  being  separated  from 
one  another  by  lymph-spaces  variable  in  size,  through  which 
communication  is  afforded  the  lymphatics  of  the  epineurium. 
Within  the  bundles  the  nerve-fibers  are  held  together  by  fibrous 
connective-tissue — the  eiidoneitrhmt.  The  epineurium  holds 
together  and  envelops  the  several  funiculi  of  the  nerve-trunk, 
the  perineurium  investing  each  funiculus,  or  primary  bundle 
of  nerve-fibers,  and  the  endoneurium  extending  among  and 
around  the  individual  fibers.  Nerve-fibers  are  divided  into  two 
classes, — which  classification  is  dependent  upon  the  presence  or 
absence  of  a  medullary  sheath  or  covering, — into  the  medullated 
or  za/iiie,  and  the  non-medullated  or  gray .  The  viedidlary  sheath, 
or  white  substance  of  Schwann,  is  a  bright,  fatty  substance  (the 
myelin)  surrounding  the  axon,  or  axis-cylinder,  the  conducting 
or  central  part  of  a  nerve-fiber.  Between  the  medullary  sheath 
and  the  axis-cylinder  there  is  present  a  small  amount  of  albumi- 
nous fluid.  Closely  surrounding  the  medullary  sheath,  and  form- 
ing the  outer  boundary  of  the  nerve-fiber,  is  the  neurilemma,  or 
sheath  of  Schwann.  Between  this  delicate,  structureless  mem- 
brane and  the  medulla  there  are  placed  at  intervals  oblong 
nuclei,  surrounded  by  protoplasm  ;  these  are  the  nerve-coipuscles . 
Besides  the  division  of   nerve-fiber  into  medullated  and    non- 


NERVES.  363 

medullated,  each  division  is  susceptible  of  further  subdivision, 
dependent  upon  the  presence  or  absence  of  the  neurilemma. 
Non-medullated  nerve-fibers  without  a  neurilemma  are  composed 
of  an  axis-cylinder  only  ;  they  are  cylindric  or  band-like  in 
form,  transparent,  and  show  faint,  longitudinal  striations.  Non- 
medullated  nerve-fibers  widi  a  neurilemma  are  composed  of  an 
axis-cylinder  surrounded  by  a  neurilemma,  and  are  homogene- 
ous throughout  their  extent. 

Medullated  nerve-fibers  are  those  which  are  pardy,  but  never 
entirely,  invested  by  a  medullary  sheath.  They  may  or  may  not 
possess  a  neurilemma;  in  the  former  instance  they  consist  of  an 
axis-cylinder  and  a  medullary  sheath  only.  The  axis-cylinder, 
or  essential  part  of  the  nerve-fiber,  is  cylindric  or  band-like,  oc- 
casionally exhibiting  a  delicate,  longitudinal  striation,  which 
appearance  is  due  to  its  being  composed  of  a  primitive  fibrillae. 

The  nerve-cells  or  ganglia-cells  are  found  in  the  ganglia  as 
well  as  along  the  course  of  the  nerves.  They  are  composed  of 
granular  or  faintly  striated  protoplasm,  inclosing  a  characteristic 
nucleus  within  which  is  a  nucleolus.  They  differ  greatly  in  form 
as  well  as  in  size,  the  spheric,  spindle-shaped,  and  irregularly 
stellate  forms  being  the  most  common.  In  the  latter  numerous 
processes  are  given  off,  forming  the  stellate  outlines.  The  cells 
are  variously  named,  according  to  the  number  of  processes.  If 
one  process  is  present,  the  cell  is  termed  a  unipolar  cell ;  if  two, 
a  bipolar ;  and  if  a  number  of  processes  exist,  they  are  named 
multipolar.  The  processes  are  of  two  varieties — the  axis-cylinder 
process  and  the  branched  protoplasmic  process.  The  various 
forms  are  most  readily  distinguished  in  the  multipolar  cells. 
The  axis-cylinder  process  is  readily  characterized  by  its  hyaline 
appearance  and  unbroken  outline.  The  protoplasmic  processes 
are  thicker,  granular,  and  striated. 

Literature. 
Stohr,  "Text-book  of  Histology,"  1896. 
Sudduth,  "American  System  of  Dentistry,"  vol.  i,  1SS6. 
Klein,  "  Elements  of  Histology,"  1889. 
Holden,  "Human  Osteology." 

Strieker,  "  Human  and  Comparative  Histology,"  vol.  i,  1870. 
Morris,  "  Human  Anatomy,"  1S96. 
Lee,  "  The  Microtomist's  Vade  Mecuni,"  1896. 


CHAPTER  II. 

THE  MUCOUS  MEMBRANE  OF  THE  MOUTH;  OF  THE  LIPS;  OF  THE 
CHEEKS;  OF  THE  GUMS;  OF  THE  ROOF  OF  THE  MOUTH,  HARD 
AND  SOFT  PALATE;  OF  THE  FLOOR  OF  THE  MOUTH;  THE 
TONGUE. 

HISTOLOGY    OF    THE    TISSUES   OF    THE    MOUTH. 

Mucous  Membrane  of  the  Mouth. — The  mucous  mem- 
brane lining  the  cavity  of  the  mouth  consists  of  two  parts — the 
epithehum  and  the  tunica  propria  ;  beneath  the  latter,  and  form- 
ing the  deeper  part  of  the  mucous  membrane,  is  the  submucosa- 


-S^     C 


V 


Fig.  245. — Vertical   Section,  Mucous   Membrane  of  the  Mouth,  Human   Embryo. 

X  150- 


T/ie  epithelutin  of  the  mouth  is  a  thick,  stratified,  squamous 
epithelium,  the  most  superficial  cells  being  scale-like  or  horn-like. 
The  cells  are  arranged  similar  to  those  in  the  epiderm,  are 
columnar  in  form,  and  contain  very  little  pigment. 

364 


MUCOUS   MEMBRANE  OF   THE   MOUTH.  365 

The  tunica  propria  is  a  somewhat  dense  feltwork  of  inter- 
lacing connective-tissue  bundles,  interspersed  with  elastic  fibers. 
The  tunica  propria  penetrates  the  epithelium  in  the  form  of 
cylindric  or  conic  papillae,  which  differ  in  length  with  the 
variation  in  the  thickness  of  the  epithelium.  As  the  mucosa  is 
usually  thickest  in  the  lips,  gums,  soft  palate,  and  uvula,  accord- 
ingly the  papillae  are  of  the  greatest  length  in  these  parts.  The 
tunica  propria  passes  into  the  submucosa  so  gradually  that  a 
positive  line  of  demarcation  can  not  be  established. 

The  siibmucosa  consists  of  a  bundle  of  fibrous  connective 
tissue  with  but  few  elastic  fibers.  This  structure  is  somewhat 
loose  in  texture  and  is  loosely  attached  to  the  underlying  perios- 
teum. Over  the  major  portion  of  the  gums  and  the  entire  hard 
palate  the  submucosa  is  attached  to  the  bones  of  the  mouth 
through  the  medium  of  their  periosteal  covering.  It  is  in  this 
loosely  constructed  tissue  that  the  glands  of  the  mucous  mem- 
brane are  situated.  These  are  for  the  most  part  branched, 
tubular,  mucous  glands.  Besides  adipose  tissue  in  the  form  of 
groups  of  fat-cells,  striped  muscular  tissue  is  present  in  the  sub- 
mucosa. In  some  parts  of  the  mouth  this  tissue  forms  a  con- 
spicuous portion — namely,  in  the  sphincter  muscle  of  the  lips 
(orbicularis  oris)  ;  also  in  the  soft  palate,  uvula,  and  pillars  of 
the  fauces. 

The  blood-supply  to  the  mucous  niembraiie  of  the  mouth  is 
principally  distributed  in  two  systems,  the  larger  vessels  to  the 
submucosa  and  the  capillaries  to  the  tunica  propria.  The  larger 
vessels  break  up  and  send  a  dense  network  of  capillaries  through 
its  substance  and  to  the  numerous  papillae  which  extend  into  the 
epithelium.  Numerous  veins  ramify  through  the  superficial  part 
of  the  tunica  propria.  The  lymphatics  form  two  networks,  the 
submucosa  giving  place  to  the  coarser  vessels,  while  the  fine 
parts  are  distributed  to  the  tunica  propria. 

Nerve-supply  to  the  Mucous  Membrane  of  the  Mouth. — In 
the  submucosa  the  medullated  nerve-fibers  form  a  wide-meshed 
reticulum,  from  which  numerous  primitive  fibrillae  pass  to  the 
tunica  propria,  where  they  terminate  or  continue  as  non- 
meduUated  nerve-fibers,  and  penetrate  the  papillae  of  the  epi- 
thelium, forminof  networks. 


366  HISTOLOGY. 

Mucous  Membrane  of  the  Lips. — Beginning  as  a  direct 
continuation  of  the  integument  or  external  covering  of  the  lips, 
the  labial  covering,  including  the  integument,  may  be  divided 
into  three  parts — namely,  a  cutaneous  portion  (best  described 
in  this  connection),  a  transitory  portion,  and  a  mucomembranous 
portion. 

The  cutaneous  portion,  covered  by  a  thin  epidermis,  consists 
of  a  double  layer  of  somewhat  flattened  epithelium.  Immedi- 
ately beneath  this  is  a  thin,  cellular,  mucous  layer,  the  cells  com- 
posing it  being  spheroid  in  form,  and  containing  nuclei  which 
are  proportionately  large.  Subjacent  to  this  is  the  cutis,  com- 
posed of  fasciculi  of  fibers  intersecting  and  closely  woven 
together,  the  principal  fibers  passing  toward  the  free  border  of 
the  mucous  membrane  covering  the  contiguous  surface  of  the 
lip.  These  fibers  are  for  the  most  part  connective-tissue  fibers, 
intermingled  with  elastic-tissue  fibers.  Numerous  small,  vascular 
papillae  are  found  upon  the  surface  of  the  cutis  ;  these  are  cylin- 
dric  or  conic  in  form,  and  project  for  some  distance  into  the 
rete  mucosa — the  lower  layers  of  living  cells  of  the  epidermis. 
Equally  distributed  at  various  depths  in  this  tissue  are  numerous 
hair-  and  sebaceous  follicles.  The  general  direction  of  the  hair- 
follicles  in  the  upper  lip  is  downward,  while  those  occupying  the 
lower  lip  are  turned  upward.  Other  than  the  distinction  noted 
by  the  difference  in  color  of  the  parts,  the  cutaneous  portion  may 
be  distinguished  from  the  transitional  mucous  membrane  by  the 
absence  of  hair-follicles  and  sebaceous  glands  in  the  latter. 

The  transitional  portion  of  the  mucous  membrane  of  the  lips 
is  outlined  externally  by  the  outer  border  of  the  red  portion  of 
the  lips,  and  internally  by  that  prominent  part  of  the  labial  con- 
vexity which  comes  in  contact  with  the  opposing  labial  fold, 
leaving  the  transitional  portion  exposed  to  view  when  the  lips 
are  in  occlusion.  The  epithelial  layer  of  this  surface  does  not 
begin  where  the  hair-follicles  cease  to  exist,  a  slight  interspace 
appearing  upon  the  cutaneous  portion  which  is  devoid  of  these 
follicles.  At  its  line  of  beginning  the  transitional  portion  of  the 
labial  mucous  membrane  is  quite  thin,  but  rapidly  increases  in 
thickness  in  passing  toward  the  mucomembranous  portion. 
Superficially  the  cells  are  much  flattened,  closely  associated  with 


MUCOUS   MEMBRANE   OF   THE   CHEEKS.  367 

one  another,  and  devoid  of  nuclei.  Tlie  cells  of  the  middle  and 
deeper  layers  are  oblong  or  spheric,  and  provided  with  irregu- 
larly shaped  nuclei.  The  chief  fibrous  tissues  of  the  transitional 
portion,  which  are  thinnest  at  the  point  where  the  hair-follicles 
cease  to  exist,  are  united  into  fle.xiform  fasciculi,  which  are  sepa- 
rated at  various  points  to  give  passage  to  numerous  minute 
blood-vessels.  The  fibrous  tissues  increase  in  thickness  as  the 
mucomembranous  portion  is  approached.  Numerous  thin  and 
somewhat  elongated  papillae  are  distributed  over  the  surface  of 
the  transitional  portion. 

The  mucomembranous  portion  of  the  mucous  membrane  of 
the  lips  includes  all  that  portion  covering  the  labial  folds  within 
the  mouth,  beginning  at  the  line  of  occlusion  on  the  contiguous 
surface  and  extending  to  the  gums.  The  epithelium  is  much 
thicker  than  that  previously  described,  and  presents  the  char- 
acteristic layers  common  to  stratified,  squamous  epithelium. 
Superficially  the  cells  are  flattened  and  tubular,  provided  with 
nuclei  of  similar  form.  In  the  middle  layer  the  cells  are  flat- 
tened and  oblong,  followed  in  the  deeper  layer  by  irregularly 
formed  nucleated  cells.  A  variety  of  fibers  make  up  the 
structure — one  class  fine  in  texture  and  united  into  fasciculi, 
intermingled  with  elastic  fibers,  together  with  another  set  of 
coarse,  strongly  looped  fibers.  Whenever  the  fibers  of  the 
tunica  propria  assume  a  definite  general  direction,  they  are  hori- 
zontal, passing  from  right  to  left  and  encircling  the  oral  aper- 
ture. The  tunica  propria  is  beset  with  numerous  conic  papillae 
which  project  into  the  epithelium  ;  these  are  longest  where  the 
epithelium  is  thickest.  The  mucous  membrane  forming  the 
labial  frena  is  covered  by  an  epithelial  layer  which  is  much 
thinner  than  that  distributed  to  other  parts  of  the  lips.  The 
fibers  in  these  are  irregularly  distributed,  and  the  papillae  are 
small  and  not  so  numerous.  The  coronary  arteries  and  their 
accompanying  veins  course  through  the  lips  near  the  junction 
of  the  transitional  with  the  mucomembranous  portion  of  the 
mucous  membrane. 

Mucous  Membrane  of  the  Cheeks. — The  mucous  mem- 
brane of  the  cheeks  presents  but  little  variation  in  its  struc- 
ture from  that  of  the  mucomembranous  portion   of  the  labial 


368 


HISTOLOGY. 


mucous  membrane.  The  buccal  epithelium  is  the  same  in 
structure  and  thickness  as  that  of  the  lips,  excepting  the  dispo- 
sition of  cells  in  the  middle  layer,  where  they  are  greater  in 
number  and  more  closely  associated,  being  somewhat  distorted 
by  contact.  The  papillae,  which  project  from  the  mucosa  into  the 
epithelium,  are  somewhat  broad  at  their  base,  with  elongated  ex- 
tremities, the  height  of  which  is  quite  variable,  in  some  instances 


-Section  through  the  Mucous   Membrane  of  the  Cheek,  showing  the 
Papill/e  of  the  Mucosa  in  Transverse  Section. 


penetrating  well  into  the  epithelium,  at  others  merely  entering 
its  deeper  layer.  At  the  anterior  portion  of  the  cheek,  or  that 
in  the  region  of  the  angle  of  the  mouth,  the  mucous  membrane, 
by  its  submucous  portion,  is  in  immediate  contact  with  the  fibers 
of  the  buccinator  muscle,  and  throughout  the  entire  surface  of 
the  cheek  it  is  closely  associated  with  this  muscle.  The  mem- 
brana  propria  is  dense  immediately  beneath  the  epithelium,  but 
as  the  buccinator  is  approached  it  becomes  much  less  so. 


MUCOUS    MEMBRANE    OF    THE    GUMS. 


369 


Mucous  Membrane  of  the  Gums. — The  mucous  mem- 
brane covering  these  parts  is,  on  account  of  the  numerous  tendi- 
nous fasciculi  which  enter  into  its  construction,  extremely  dense 
and  tough,  these  characteristics  being  more  strongly  manifest 
here  than  in  any  other  portion  of  the  oral  mucous  membrane. 
These  qualities  are  especially  pronounced  about  the  gingival 
margins  and  over  the  major  portion  of  the  alveolar  walls,  being 


Fig.  247. — Section  through  thk  Gums,  showing  Epitheliuji  and  Basement 
Membrane. 


closely  bound  down  to  the  bone  by  direct  prolongations  of  the 
tendinous  fasciculi  of  the  periosteum  which  penetrate  the  mem- 
brane. As  the  gingival  mucous  membrane  passes  into  that  of 
the  lips  and  cheeks  it  gradually  becomes  less  dense.  The  epi- 
thelium of  the  mucous  membrane  of  the  gums  is  composed  of 
lamina  of  tessellated  and  ribbed  cells.  The  superficial  cells  are 
the  flattened  cells  of  pavement  epithelium  ;  subjacent  to  this 
they  become  thicker  and  deeply  ribbed,  while  the  deepest  cells 
24 


370  HISTOLOGY. 

are  conic  or  cylindric  with  conic  extremities.  The  tissue  com- 
posing the  tunica  propria  is  made  up  of  flattened  fasciculi  of 
connective  tissue,  the  fibers  of  which  run  parallel  with  one 
another.  Numerous  elastic  fibers  are  also  present.  Three  sets 
of  fibers  are  to  be  distinguished  in  the  mucous  membrane  of 
the  gums — those  which  run  vertically,  those  which  pass  in  a 
horizontal  direction,  and  those  which  radiate  or  are  distributed 
fan-like.  Of  the  first  named,  the  fibers  extend  from  above 
downward ;  in  the  second  class  they  pass  from  right  to  left  par- 
allel with  the  surface  ;  the  third  class,  including  those  fibers 
which  are  reflected  from  the  alveolodental  membrane,  are  dis- 
tributed in  fasciculi  about  the  margins  of  the  alveoli. 

Mucous  Membrane  of  the  Roof  of  the  Mouth. — Hard 
Palate. — The  mucous  membrane  covering  the  hard  palate  is,  in 
very  many  respects,  dissimilar  to  that  surrounding  the  necks  of 
the  teeth  and  forming  the  palatogingival  margins.  Like  the 
mucous  membrane  of  the  gums,  that  overlying  the  hard  palate 
is  dense  and  tough.  The  papillee  of  the  tunica  propria,  which 
penetrate  the  epithelium,  are  not  so  numerous  as  those  upon  the 
gums.  In  the  posterior  third  of  the  hard  palate  they  are  some- 
what more  numerous  and  generally  a  little  more  prominent  than 
those  in  the  anterior  portion.  In  the  median  raphe  and  over  the 
rugae,  the  papillse  are  especially  sparingly  distributed.  The  epi- 
thelium is  of  the  pavement  variety,  somewhat  thinner  in  front 
than  behind,  the  cells  being  more  freely  distributed  at  some 
points  than  at  others.  The  mucous  membrane  of  the  hard 
palate  is  less  in  thickness  anteriorly  than  posteriorly.  The  dis- 
tribution of  the  fibers  is  such  that  they  radiate  from  the  alveolar 
borders  toward  the  center  of  the  palate,  the  anterior  fibers 
passing  obliquely  backward,  while  those  from  the  lateral  walls 
pass  parallel  with  one  another  to  the  median  line.  For  the  most 
part  the  fibers  are  broad  and  form  a  plexus  between  the  epithe- 
lium and  the  submucous  tissue.  The  submucous  tissue  is 
sparingly  distributed  over  the  central  portion  of  the  hard  palate, 
but  laterally  is  somewhat  more  abundant,  containing  a  few 
fat-cells. 

Soft  Palate,  Uvula,  and  Fauces. — Passing  backward  from 
the  posterior  margin  of  the  hard  palate,  the  mucous  membrane 


MUCOUS   MEMBRANE   OF   THE   FLOOR  OF  THE   MOUTH.  371 

overlies  the  fibrous  aponeurosis  of  the  soft  palate  and  its  median 
and  lateral  prolongations — the  uvula  and  pillars  of  the  fauces. 
The  epithelium  is  of  the  laminated  pavement  variety,  with  the 
deeper  cells  larger  than  those  placed  superficially.  The  sub- 
stance of  the  mucous  membrane  is  composed  of  fasciculi  of 
connective  tissue,  intermingled  with  a  plexus  of  elastic  fibers. 
The  fibers  are  distributed  in  three  principal  directions — from 
side  to  side,  or  horizontally,  longitudinally,  and  obliquely.  The 
oblique  fibers  are  instrumental  In  forming  the  submucous  tissue 
of  both  the  soft  palate  and  uvula.  Numerous  conic  papillae 
project  from  the  tunica  propria  into  the  epithelium  ;  these  are 
larger  and  more  numerous  on  the  uvula  than  on  the  soft  palate. 
The  tunica  propria  Is  somewhat  variable  in  thickness  to  accom- 
modate the  glands,  which  are  more  or  less  numerous,  and  pres- 
ent In  greater  numbers  in  one  Instance  than  In  another.  In 
general  the  membrane  as  a  whole  is  thinnest  along  the  margin 
of  the  hard  palate,  gradually  increasing  In  thickness  as  the  free 
border  is  approached.  The  folds  of  mucous  membrane  forming 
the  pillars  of  the  fauces  present  no  peculiarity  differing  from 
that  of  the  soft  palate,  save  a  more  generous  supply  of  elastic 
fibers. 

Mucous  Membrane  of  the  Floor  of  the  Mouth. 

The  Tono-jie. — The  entire  unattached  surface  of  the  tongue  is 
covered  by  a  reflection  of  the  mucous  membrane  of  the  floor  of 
the  mouth.  In  this  organ  the  general  structure  of  the  mucous 
membrane  does  not  vary  from  that  of  other  oral  mucous  mem- 
brane, being  composed  of  an  epithelium,  a  tunica  propria,  and  a 
submucosa.  The  mucous  membrane  covering  the  dorsum  of 
the  tongue  presents  special  characteristics,  which  differ  from  that 
of  the  under  surface  and  the  floor  of  the  mouth  in  general.  In 
the  former  location  the  papillary  elevations  of  the  tunica  propria 
are  conspicuously  developed,  and  with  their  covering  of  strati- 
fied, scaly  epithelium  cause  the  peculiar  furred  appearance. 
Three  classes  of  papillae  are  distinguished,  named,  In  accordance 
with  their  iorm,  filiform  paplllje,  fungiform  papillae,  and  circum- 
vallate  papillae. 

The  filiform  papilla;,  which  are  very  numerous  over  the  entire 


HISTOLOGY. 


dorsum  and  sides  of  the  tongue,  are  conic  and  frequently  pro- 
longed into  numerous  horn-like  processes,  known  as  secondary 


Fig.  248. — Longitudinal  Section  through  Mucous  Membrane  of  the  Human 
Tongue.     X  20. 


Fig.  249 — Longitudinal  Section  of  the  Mucous  Membrane  of  the  Human  Tongue, 
showing  the  Fungiform  and  Secondary  Papill.^.     X  80. 


papillae.  As  elevations  from  the  tunica  propria  they  are  com- 
posed of  well-defined  fibrillated  tissue,  intermingled  with  numer- 
ous  elastic    fibers.     The  pavement    epithelial   cells    are   found 


MUCOUS    MEMBRANE   OF  THE    FLOOR   OF   THE   MOUTH.  373 

overlapping  one  another,  and   provided  witli  processes  which 
project  beyond  the  papillae. 

The  finigiform  papillce  are  also  distributed  over  the  entire 
dorsum  and  sides  of  the  tongue,  but  are  somewhat  less  numerous 
than  the  filiform  variety.  They  appear  as  well-defined  eleva- 
tions, and  are  connected  with  the  tunica  propria  by  a  constricted 
portion  or  neck.  The  entire  free  or  rounded  surface  of  these 
papillae  is  beset  with  secondary  papillee.  The  epithelium  is 
slightly  thinner  than  that  over  the  filiform  papillae,  this  being  the 
principal  distinguishing  feature.  The  numerous  capillaries  pro- 
duce a  rich  red  color,  plainly  observable  through  the  transparent 


Fig.  250. — Section  through  Epithelium,  Near  the  Tip  of  the  Tongue.     X  4°- 

epithelium.     Connective-tissue    bundles  make  up  the  bulk    of 
these  papillae,  few  elastic  fibers  being  present. 

The  cii-cumvallate  papilla  are  placed  on  the  posterior  portion 
of  the  dorsum  of  the  tongue,  and  are  few  in  number  (eight  to 
sixteen).  They  are  much  larger  than  those  already  described, 
and  in  general  resemble  modified  fungiform  papillae.  They 
are  flattened  and  broad,  and  differ  from  the  fungiform  by  having 
a  circular  furrow  or  wall  surrounding  them.  Secondary  papillae 
are  present  on  the  free  surface  only,  the  sides,  and  in  some 
instances  the  walls  surrounding  them,  being  occupied  by  the 
end  organs  of  the  special  sense  of  taste — the  taste-buds.     Other 


374  HISTOLOGY. 

taste-buds  are  found  upon  the  lateral  margins  of  the  tongue 
posteriorly,  nestled  in  a  group  of  parallel  folds  of  mucous  mem- 
brane— the  papillae  foliata.  The  connective  tissue  within  these 
papillae  is  similar  to  that  in  the  fungiform  papillae.  On  other 
parts  of  the  tongue,  or  those  portions  not  occupied  by  these 
specially  constructed  papillae,  the  epithelium  is  similar  to  that  in 
other  parts  of  the  mouth.  The  tunica  propria  is  less  in  thick- 
ness in  and  about  the  tip  of  the  tongue,  and  is  intimately  con- 
nected with  the  subjacent  muscular  structure.  As  the  root  of 
the  organ  is  approached,  the  tunica  propria  becomes  thicker 
and  more  dense.  The  submucosa  Is  especially  intimately  con- 
nected with  the  underlying  parts  at  the  margins  and  tip  of  the 
tongue.  The  extreme  portion  of  the  root  of  the  tongue  has  its 
mucous  membrane  particularly  modified  by  a  special  aggrega- 
tion of  adenoid  tissue — developed  lymph-nodules.  These  are 
large  and  readily  perceptible  to  the  naked  eye.  They  are  pro- 
vided with  a  central  opening,  which  dips  down  into  a  well- 
defined  vault  or  crypt,  which  is  lined  by  a  reflection  of  the 
stratified  oral  epithelium. 

Blood-supply  to  the  Mucous  Membrane  of  the  Mouth. 
— The  oral  mucous  membrane  derives  its  supply  of  blood  froni 
numerous  branches  of  the  external  carotid  artery — namely,  the 
superior  and  inferior  coronary,  buccal,  lingual,  transverse  facial, 
pterygopalatine,  and  the  alveolar.  Entering  the  submucosa, 
the  minute  terminal  branches  of  these  arteries  are  distributed 
parallel  to  the  surface,  and  by  anastomosis  form  plexuses  from 
which  other  minute  branches  are  given  off  to  supply  the  papillae 
of  the  tunica  propria.  After  coursing  through  the  papillae  the 
blood  is  discharged  into  a  similar  venous  plexus,  and  thus 
conveyed  from  the  parts.  In  a  like  manner  the  mucous  mem- 
brane and  papillae  of  the  tongue  are  supplied,  branches  of  the 
lingual  artery  conveying  the  blood  to  the  parts.  The  dorsalis 
linguae  supplies  the  mucous  membrane  of  the  dorsum  of  the 
tongue  and  pillars  of  the  fauces,  while  the  ranine  artery  by  its 
minute  branches  supplies  the  remaining  mucous  membrane. 
Each  papilla  is  entered  by  two  or  more  arterial  terminals,  which 
divide,  anastomose,  and  finally  send  off  capillary  branches  to 
the  secondary  papillae. 


NERVE-SUPPLY  TO  THE  MUCOUS  MEMBRANE  OF  THE  MOUTH.       375 

Nerve-supply  to  the  Mucous  Membrane  of  the  Mouth. 

— The  distribution  of  the  nerve-fibers  to  the  oral  mucous  mem- 
brane is  approximately  similar  in  all  parts.  The  fibers,  which 
are  of  the  medullated  variety,  are  first  distributed  to  the  sub- 
mucosa,  forming  a  wide-meshed  reticulum.  From  this  fibers 
are  given  off  to  the  tunica  propria,  terminating  in  end-bulbs, 
or,  after  losing  their  medullary  sheath,  are  distributed  to  the 
epithelium,  where  their  free  extremities  lie  between  the  epi- 
thelial cells.  The  nerves  of  the  mucous  membrane  of  the 
tongue  (the  glossopharyngeal  and  lingual  branch  of  the  fifth) 
may  have  their  endings  similar  to  those  in  other  parts  of  the 
mouth,  or  they  may  be  intimately  associated  with  the  taste-buds. 

Literature. 
Legros  and  Magitot,  1S80. 
Klein,  "  Structure  of  the  Oral  Lips,"  1S68. 

Sebastian,  "Anatomy  and  Physiology  of  the  Labial  Glands,"  1842. 
KoUiker,  "  Mikroskopische  Anatomie." 
Kirke,  "Physiology." 

Strieker,  "  Human  and  Comparative  Histology." 
Stohr,  "  Text-book  of  Histology,"  1896. 


CHAPTER  III. 

GLANDS  AND  DUCTS  OF  THE  MOUTH;  OF  THE  LIPS;  OF  THE  CHEEKS 
OF  THE  HARD  AND  SOFT  PALATES;  OF  THE  TONGUE.— THE  SALI- 
VARY GLANDS. 

GLANDS  AND   DUCTS. 

Glands  of  the  Mouth. — The  glands  of  the  mouth,  Hke  the 
glands  of  other  parts  of  the  body,  are  composed  almost  en- 
tirely of  epithelium,  and  may,  therefore,  be  classed  with  the 
epithelial  tissues.  Glands  exist  in  two  principal  forms — tubular 
and  saccular  (alveolar).  The  former  occur  either  singly  or  in 
groups,  and  are  further  subdivided  into  simple  tubular  and  com- 
pound tubular  glands.  A  like  condition  is  present  in  the  saccular 
glands  and  similar  terms  are  employed  to  qualify  them — simple 
saccular  glands  and  compound  saccular  glands. 

A  simple  tubular  gland  \s  one  composed  mainly  of  a  simple 
tube-like  structure  ;  2l  compotmd  tubular  gland  x?,  one  composed 
of  a  number  of  smaller  tubes  emptying  into  a  single  duct. 

A  simple  saccular  gland  is  one  iormed  by  a  sacculation  of 
serous  or  mucous  membrane  into  a  single,  simple  sac,  or  by 
branched  saccules  having  an  excretory  duct  (alveolar  system) ; 
a  compound  saccidar  gland  is  composed  of  a  combination  of 
branched  saccules. 

In  the  larger  glands  a  sheath  is  formed  by  the  surrounding 
connective  tissue,  from  which  numerous  septa  are  given  off  to 
the  interior  of  the  gland,  dividing  it  into  compartments  varying 
in  size.  These  are  known  as  gland-lobides.  The  connective- 
tissue  walls  of  the  gland-lobules  carry  the  larger  blood-vessels 
and  nerves.  Most  glands  are  divided  into  two  essential  parts, 
— the  gland-follicle  and  the  excretory  duct, — the  former  being 
specialized  for  the  secretory  function,  while  the  latter,  by  com- 
municating with  the  surface,  conveys  the  secreted  substance  to 
that  point. 

77^1?  gland-follicles  are  composed  of  a  layer  of  gland-cells, 
376 


THE   GLANDS   OF   THE   LIPS,  377 

usually  simple  in  character,  surrounding  the  follicular  walls. 
External  to  these  is  a  specially  modified  connective  tissue,  form- 
ing the  basement  membrane,  or  membrana  propria.  The 
appearance  of  the  gland-cells  and  their  nuclei  is  continually 
changing,  being  thus  influenced  by  their  functional  activity. 

The  excretory  ducts  consist  of  a  wall  of  connective  tissue  and 
elastic  fibers,  lined  by  a  columnar  epithelium,  either  simple  or 
stratified.      In  some  instances  the  arrangement  of  the  e.xcretory 


Fig.  251. — Section  through  the  Glandular  Tissue  ok  the  Tongue.     >(  4°- 


ducts  is  much  complicated,  being  divided  into  secretory  tubes, 
which  in  turn  are  subdivided  into  smaller  tubules — intercalated 
tubes. 

The  Glands  of  the  Lips  {Labial  Glands). — The  glands  of 
the  lips  are  situated  in  the  submucosa,  and  are  first  observed 
immediately  within  the  line  of  labial  occlusion,  at  which  point 
the  thickness  of  the  epithelium  becomes  somewhat  definite  and 
general.  These  glands  are  variable  in  size,  but  all  are  suffi- 
ciently large  to  be    observed    without   the    aid    of  the    micro- 


37S  HISTOLOGY. 

scope.  They  are  of  the  compound  tubular  variety,  and  com- 
municate with  the  surface  through  an  excretory  duct,  which 
throughout  the  greater  part  of  its  extent  is  Hned  with  stratified, 
scaly  epithelium.  In  passing  from  the  surface  toward  the  gland- 
follicle,  the  main  duct  takes  a  spiral  course  obliquely  through 
the  tunica  propria,  and  upon  reaching  the  submucosa  gives  off 
numerous  branches  and  twigs  which  terminate  in  the  individual 
acini.  The  larger  branches  from  the  main  duct  are  lined  with 
stratified  squamous  epithelium,  while  the  smaller  twigs  are  pro- 
vided with  columnar  epithelium.  In  many  instances  the  main 
excretory  duct,  in  its  passage  through  the  tunica  propria,  receives 
the  principal  duct  from  small  accessory  ducts.  The  framework 
of  the  labial  glands  is  formed  by  the  flexiform  tissue  composed 
of  fasciculi  of  the  fine  connective-tissue  fibers  belonging  to  the 
submucous  layer,  together  with  delicate,  coiled  elastic  fibers. 
This  framework  gives  support  to  a  minute  system  of  capillaries 
and  small  nerve-fibers  supplying  the  acini.  The  acini  are  so  ar- 
ranged that  those  belonging  to  a  large  duct  are  united  into  a 
lobule  by  the  submucous  connective-tissue  fasciculi,  and  these  in 
turn  are  formed  into  lobes.  By  a  continuation  of  the  same  fas- 
ciculi and  fibers  which  limit  a  lobe,  and  in  the  meshes  of  which 
the  acini  are  situated,  a  sheath  to  the  excretory  duct  is  formed. 
Besides  the  branched,  tubular,  mucous  glands  of  the  lips,  there 
are  occasionally  found,  at  the  edges  of  the  lips,  sebaceous  glands. 
The  Glands  of  the  Cheeks  {Buccal  Glands,  Molar  Glands). 
— The  glands  of  the  cheek  are  also  situated  in  the  submucous 
layer  of  the  mucous  membrane.  They,  like  the  labial  glands, 
are  of  the  compound  tubular  variety,  and  when  microscopically 
examined  are  found  to  be  similar  in  structure.  They  are  some- 
what larger  than  the  labial  glands  and  proportionately  less 
numerous.  The  chief  duct  from  each  of  these  glands  usually 
opens  with  a  narrow  mouth  on  the  surface  of  the  oral  mucous 
membrane,  and  in  its  passage  through  the  tunica  propria  takes 
a  vertical  or  oblique  direction.  In  the  submucosa  the  chief  duct 
branches  into  two  or  more  smaller  ducts,  taking  up  alveoli.  As 
the  buccal  glands  are  somewhat  larger  than  the  glands  of  the 
lips,  they  are  composed  of  a  greater  number  of  ducts  and 
alveoli. 


THE  GLANDS  OF  THE  TONGUE.  379 

The  Glands  of  the  Hard  and  Soft  Palate  {Palatal  Glands). 
— The  mucous  glands  of  the  hard  palate  are  situated  in  the  sub- 
mucosa  and  closely  associated  with  the  periosteum.  They  are 
compound  tubular  glands,  and  in  all  essential  particulars  are 
similar  to  the  labial  and  buccal  glands.  They  are  quite  numer- 
ous (200  to  300),  isolated  in  the  anterior  portion,  but  are 
grouped  into  a  single  row  or  into  two  rows  posteriorly.  The 
glands  are  freely  distributed  in  each  lateral  half,  but  are  absent 
at  the  median  line. 

In  die  soft  palate  the  glands  are  of  the  same  character,  some- 
what variable  in  size,  the  largest  being  found  in  the  uvula.  The 
excretory  ducts  from  these  glands  vary  in  diameter,  in  the 
nature  of  their  fibrous  structure,  and  in  the  direction  taken  in 
passing  to  the  surface.  Over  the  surface  of  the  soft  palate  the 
mouth  of  these  ducts  are  represented  by  minute  orifices  slightly 
smaller  than  die  body  of  the  duct,  but  in  the  uvula  the  opposite 
condition  is  present,  the  mouth  of  the  duct  being  wider  than  the 
body.  The  course  taken  by  the  excretory  duct  is  seldom  a 
direct  one,  but  after  receiving  all  tributary  branches  passes 
obliquely  through  the  tunica  propria,  and  before  entering  the 
epithelium  turns  at  an  abrupt  angle,  and  so  continues  until  the 
surface  is  reached.  The  ducts  are  lined  by  a  simple  columnar 
epithelium,  which  in  some  instances  is  ciliated  ;  the  walls  of  the 
tubes  consist  of  gland-cells  and  a  structureless  membrana  pro- 
pria. In  some  instances  the  surface  epithelium  may  be  reflected 
for  a  short  distance  and  partly  serve  in  the  capacity  of  a  lining 
to  the  tubular  walls. 

The  Glands  of  the  Tongue  {Lingual  Glands). — In  this 
organ  two  varieties  of  glands  are  found,  occurring  both  in  the 
mucous  membrane  and  in  the  superficial  muscular  strata,  being 
principally  distinguished  by  the  nature  of  their  secretions.  The 
gland-cells  of  the  one  set  are  mucigenous,  secreting  mucin  ; 
these  are  the  mucous  glands.  The  other  set  is  productive  of  a 
serous  fluid,  thin,  watery,  and  containing  albumin  ;  these  are 
the  serous  glands. 

The  mucous  glands  of  the  tongue  are  found  along  the  lateral 
margins  and  over  the  root  of  the  organ,  being  most  numerous  in 
the  latter  situation.     They  are  of  the  compound  tubular  variety. 


38o 


HISTOLOGY. 


and  in  most  particulars  are  identical  with  the  mucous  glands  of 
other  parts  of  the  oral  cavity.  The  ducts  are  lined  with  ciliated 
columnar  epithelium,  and  the  walls  of  the  duct  consist  of  a 
homogeneous  membrana  propria  and  gland-cells.  The  glands 
occupying  the  root  of  the  tongue  are  frequently  found  with 
their  excretory  ducts  opening  into  the  follicular  crypts.  The 
tubules  consist  of  a  structureless  membrana  propria  and  num- 
erous gland-cells,  the  latter  varying  in  appearance  according  to 
their  function  or  functional  activity.  The  crypts  of  the  folli- 
cles  constitute    reservoirs  for    the  acinous    glands,  and   these 


«Si5s5^ 


Fig.  252. — Section  through  Base  of  Tongue,  showing  Serous  Glands. 


receptacles  frequently  extend  for  some  distance  beneath  the 
surface,  receiving  at  various  points  the  main  excretory  ducts 
from  the  mucous  glands.  These  saccular-like  reservoirs  are 
lined  by  a  well-defined  capsule  surrounded  by  a  fibrous  sheath, 
internal  to  which  is  an  epithelial  covering,  a  prolongation  of 
the  common  epithelium  of  the  mouth.  Between  these  two 
layers  are  a  number  of  minute,  closed  lymph-follicles  placed  in  a 
single  layer.  The  mucous  glands  on  the  lateral  walls  of  the 
tongue   are,   for    the   most   part,    situated    near   the   middle   or 


THE    SALIVARY    GLANDS.  381 

posterior  portion.  The  ducts  from  these  glands  usually  open 
directly  toward  the  cheek,  but  in  rare  instances  they  pass 
obliquely  downward  and  open  near  the  proper  floor  of  the 
mouth.  At  the  tip  of  the  tongue,  burled  beneath  the  mucous 
membrane  and  some  of  the  muscular  fibers,  may  be  found  a 
pair  of  mucous  glands  (Nuhn's)  which  open  by  free  orifices 
on  the  under  surface.  At  the  root  of  the  tongue,  flat,  len- 
ticulated  elevations  of  the  mucous  membrane  are  present, 
beneath  which  is  imbedded  conglobate,  glandular  substance. 
These  show  a  central  orifice  leading  to  a  small  pit  lined  with 
tessellated  epithelium. 

The  serous  glands  of  the  ioiigue  are  compound  tubular  glands, 
and  are  found  in  the  region  of  the  circumvallate  papillse,  closely 
associated  with  the  taste-buds.  The  excretory  ducts,  lined  with 
a  simple  or  stratified  columnar  epithelium,  the  latter  sometimes 
ciliated,  open  near  the  base  of  the  papilla,  or  between  the 
papilla  and  its  wall.  The  tubules  are  similar  to  those  in  the 
mucous  glands,  consisting  of  a  delicate,  structureless  membrana 
propria  and  gland-cells.  The  gland-cells  are  composed  of  a 
frail,  transparent  protoplasm,  containing  rounded  nuclei. 

The  Salivary  Glands. — The  parotid,  submaxillary,  and 
sublingual  glands  each  consists  of  an  excretory  duct,  branch- 
ing frequently  in  a  tree-like  manner  into  smaller  ducts,  lined 
throughout  with  a  layer  of  epithelial  cells.  From  the  smaller 
ducts  terminal  branches  are  given  off,  which  in  turn  are  lined 
with  epithelium.  The  other  portions  of  the  glands  are  invested 
by. columnar  epithelium,  and  arranged  like  grapes  about  the 
main  excretory  duct,  and  consequently  belong  to  the  group  of 
racemose  glands.  The  terminal  branches  or  alveoli  attached  to 
the  smaller  excretory  ducts  are  so  numerous  that  they  become 
much  compressed  from  pressure,  and  the  grape-like  appearance 
is  more  or  less  destroyed,  and  but  little  space  is  left  for  inter- 
stitial tissue.  Each  gland  is  inclosed  in  a  fibrous  connective- 
tissue  capsule,  and  from  this  numerous  septa  of  fibrous 
trabeculae  pass  to  the  interior  and  divide  the  glandular  sub- 
stance, first  into  lobes,  these  being  subdivided  into  lobules,  the 
lobules  by  further  subdivision  forming  the  alveoli.  The  glandu- 
lar connective  tissue  is  loose  in  texture,  containing  many  elastic 


382  HISTOLOGY. 

fibers  and  lymphoid  cells.  Fine  bundles  of  fibrous  tissue, 
together  with  branched  connective-tissue  corpuscles,  constitute 
the  connective-tissue  matrix  between  the  alveoli. 

The  Ducts. — Entering  the  interior  of  the  gland,  the  chief  duct 
divides  into  a  number  of  large  branches,  one  of  which  passes 
to  each  lobe,  each  of  these  giving  off  several  branches  which 
connect  with  the  several  lobules.  Upon  close  examination  the 
central  tube  of  each  lobule  is  observed  to  throw  off  several 
small  tubes — the  intralobular  tubes.  Following  these  are  the 
intermediate  tubules,  which  continue  into  the  terminal  compart- 
ments. The  chief  excretory  duct  consists  of  a  double  layer  of 
cylindric  epithelium  and  fibro-elastic  cartilage.  Close  beneath 
the  epithelium  is  a  compact  membrana  propria.  The  intralob- 
ular tubes  are  each  provided  with  a  distinct  lumen.  The  walls 
are  composed  of  a  membrana  propria  lined  by  a  layer  of 
columnar  epithelium,  the  cells  of  which  contain  a  central  round 
nucleus. 

The  Parotid  Gland. — The  distinguishing  histologic  feature 
in  this  gland  is  found  in  its  excretory  duct  (Stenson's  duct), 
Avhich  is  provided  with  a  membrana  propria,  especially  broad 
and  compact,  placed  immediately  beneath  the  epithelium.  The 
duct  is  composed  of  a  double  layer  of  cylindric  epithelium  and 
fibrous  tissue,  intermingled  with  elastic  fibers.  The  main  duct 
divides  and  passes  into  the  intralobular  tubes,  beyond  which 
are  the  intermediate  tubules.  The  intralobular  tubes  are  lined 
by  columnar  cells,  while  the  intermediate  tubules  are  lined  by 
elongated,  spindle-shaped  cells.  The  salivary  cells  lining  the 
acini  are  different  in  character  from  those  in  the  submaxillary 
and  sublingual  glands.  The  parotid  gland  is  a  true  salivary 
gland,  and  the  serous  gland-cells  composing  its  epithelial  lining 
are  disposed  in  a  single  layer.  The  cells  are  columnar  or  pyra- 
midal in  form  and  composed  of  a  dense  protoplasm,  containing 
a  spheric  nucleus. 

The  Submaxillary  Gland. — The  excretory  duct  (Whar- 
ton's duct),  like  the  main  duct  of  the  parotid  gland,  is  composed 
of  a  double  layer  of  columnar  epithelium,  external  to  which  is 
a  layer  of  cellular  connective  tissue,  the  whole  being  surrounded 
by  a  thin  stratum  of  muscular  fibers  placed  longitudinally.    The 


NERVE  SUPPLY    TO   THE   SALIVARY   GLANDS.  383 

intralobular  tubes  are  lined  by  a  specialized,  elongated,  cylindric 
epithelium,  which,  in  the  intermediate  tubules,  becomes  clothed 
with  cubic  cells.  The  acini  are  lined  either  with  serous  gland- 
cells  similar  to  those  lining  the  acini  of  the  parotid  gland  or  with 
mucous  gland-cells,  the  former  being  most  constant  in  their 
presence.  The  two  kinds  of  acini  are  uninterruptedly  connected. 
In  most  instances  there  are  but  a  few  mucous  acini  present 
within  the  lobule,  but  occasionally  they  are  found  in  abundance. 
The  submaxillary  is  a  mixed  or  mucosalivary  gland. 

The  Sublingual  Gland. —  The  excretory  duct  (Rivini's 
duct)  is  similar  in  structure  to  the  chief  excretory  duct  of  the 
submaxillary  gland.  The  intralobular  tubes  are  lined  with  col- 
umnar epithelium.  The  intermediate  tubules  are  not  positively 
known  to  exist,  and  it  is  quite  probable  that  the  intralobular 
tubes  pass  directly  into  the  terminal  compartments.  The  acini 
are  composed  of  a  membrana  propria  and  gland-cells,  both 
mucous  and  serous.  The  former  are  much  more  numerous  than 
in  the  acini  of  the  submaxillary.  The  membrana  propria  is 
composed  of  stellate  connective-tissue  cells.  This  gland  is  also 
a  mixed  or  mucosalivary  gland. 

Blood-vessels  and  Lymphatics  in  the  Salivary  Glands. 
— The  lobules  of  the  salivary  glands  are  richly  supplied  with 
blood-vessels.  The  many  arterial  branches  break  up  into 
numerous  capillaries,  which,  forming  a  dense  network,  sur- 
round the  acini,  being  supported  by  the  interalveolar  connec- 
tive tissue.  The  lymphatic  vessels  accompanying  the  intralobu- 
lar tubes  are  in  communication  with  numerous  lymph-spaces 
which  exist  between  the  interalveolar  connective  tissue  and  the 
walls  of  the  acini.  The  substance  of  the  gland  is  further  sup- 
plied with  blood  by  numerous  plexuses  of  lymphatics  which  are 
carried  or  supported  by  the  interlobular  connective  tissue. 

Nerve-supply  to  the  Salivary  Glands. — The  nerve-fibers 
distributed  to  the  salivary  glands  are  both  of  the  medullated  and 
non-medullated  variety,  and  other  nerve-tissue  in  the  form  of 
ganglion-cells  is  present.  The  medullated  nerve-fibers  are 
abundantly  numerous,  and  are  distributed  to  all  parts  of  the 
gland.  In  many  respects  the  fibers  are  peculiarly  constructed. 
They  are    extremely    delicate,   made    so    by    the    frail    nature 


384  HISTOLOGY. 

of  their  medullary  sheath  ;  they  divide  and  give  off  so  many 
branches  as  to  almost  give  them  a  feathery  fineness.  This 
peculiarity  is  especially  noticeable  toward  their  extremities, 
where  the  fibers  lie  between  the  alveoli  and  give  off  minute 
branches  in  all  directions.  The  nerve-fibers  are  placed  in  close 
relation  to  the  tubes  and  tubules,  which  they  freely  encircle  ; 
they  perforate  the  membrana  propria  and  break  up  into  finer 
subdivisions,  from  which  they  are  distributed  to  the  exterior  of 
the  epithelial  cells.  In  the  alveoli  two  kinds  of  nerve  termina- 
tions are  found.  The  primitive  fibers  branch  between  the  alveoli 
and  are  distributed  to  the  membrana  propria,  upon  entering 
which  numerous  branches  are  thrown  off  which  pass  to  the 
epithelial  cells  beneath.  The  non-medullated  fibers,  which  are 
much  less  numerous,  are  composed  of  an  extremely  delicate 
fasciculi  of  transparent  fibers  resembling  axis-cylinders,  and 
invested  by  a  sheath  of  connective-tissue  cells  containing  nuclei. 
The  distribution  of  these  fibers  is  similar  to  the  medullated  fibers, 
encircling  the  tubes  and  penetrating  the  membrana  propria, 
being  similarly  distributed  to  the  alveoli. 

Literature. 

Stohr,  "  Text-book  of  Histology,"  1896. 

Strieker,  "Human  and  Comparative  Histology,"  vol.  i,  1870. 

Klein,  "  Elements  of  Histology,"  1889. 

Sebastian,  "  Recherches  anatomique,  physiologiques,  pathologiques,  les  Glans 
Labiales,"  1842. 

Ward,  "On  Salivary  Glands,"  Tood's  "  Cyclopedia  of  Anatomy  and  Physi- 
ology." 

Klein  and  Vernon,  Strieker's  "  Human  and  Comparative  Anatomy,"  vol.  i, 
ehap.  XVI. 

Sudduth,  "  Embryology  and  Dental  Histology,"  "  Ameriean  System  of  Den- 
tistry," vol.  I,  part  III. 

Brubaker,  "  Transactions  Odontologieal  Soeiety  of  Pennsylvania,"  i88g-'95. 

Todd  and  Bowman,  "Physiological  Anatomy,"  vol.  i. 

Szontagh,  "  Essays  on  Minute  Anatomy  of  Hard  Palate  in  Man,"  1866. 


CHAPTER  IV. 

MUSCULAR  TISSUES  OF  THE  MOUTH;  OF  THE  LIPS;   OF  THE  CHEEKS; 
OF  THE  SOFT  PALATE;  OF  THE  TONGUE. 

MUSCULAR  TISSUES   OF  THE   MOUTH. 

Muscular  Tissues  of  the  Lips. — The  minute  bundles 
forming  the  fasciculi  of  the  oral  sphincter  muscle — the  orbicu- 
laris oris — are  distributed  between  the  submucosa  of  the  muco- 
membranous  portion  and  the  subcutaneous  tissue  of  the 
cutaneous  portion  of  the  lips.  The  muscular  fibers  radiate  in 
three  principal  directions  upon  either  side  of  the  median  line  : 
from  the  angle  of  the  mouth  toward  the  median  line,  and  from 
the  fleshy  slips  of  the  maxilla  and  mandible — the  musculi  incisivi. 
As  the  fibers  from  the  angle  of  the  mouth  pass  to  the  sub- 
stance of  the  lip,  they  are  arranged  in  a  laminated  manner. 
When  the  median  line  is  reached,  one  set  of  fibers  terminate 
somewhat  abruptly  in  the  subcutaneous  tissue,  another  set  is 
continued  beyond  the  median  line  and  attached  to  the  cutis  of 
the  opposite  side,  while  a  third  set,  without  crossing  the  median 
line,  is  attached  to  the  incisive  fossae  of  the  maxilla  and 
mandible.  The  numerous  muscular  fibers  of  the  internal 
labial  or  mucomembranous  portion,  and  the  external,  facial,  or 
cuticular  portion,  penetrate  the  parts  and  terminate  in  close 
proximity  to  the  epithelium  or  to  the  base  of  the  papillae.  Deli- 
cate, hair-like  fibers  which  are  continuous  with  the  sarcolemma 
slightly  penetrate  the  cutis  and  membrana  propria.  A  few  of 
the  fibers,  which  may  be  classed  with  the  terminals  of  the  out- 
running muscles  from  the  lips,  are  arranged  in  a  number  of 
fasciculi  In  the  subcutaneous  portion,  pass  through  the  fasci- 
culi of  the  orbicularis  oris,  reach  the  submucous  tissue,  where 
they  cross  and  recross  one  another,  and  finally  pass  into  the 
membrana  propria,  where  they  end  in  fan-like  terminals.  The 
fasciculi  of  the  orbicularis  differ  somewhat  in  the  upper  and 
lower  lips  ;  in  the  former  the  bundles  are  strongly  developed 
25  3S5 


386  HISTOLOGY. 

toward  the  angle  of  the  mouth,  while  in  the  latter  the  median 
bundles  are  the  strongest.  The  labial  muscular  tissues  are  of 
the  transversely  striated  variety.  The  fibers  are  cylindric  in 
form,  having  rounded  or  pointed  extremities  in  the  interior,  and 
broad  or  flattened  ends  where  they  come  in  contact  with  the 
periosteum.  When  examined  with  a  high  power  each  fiber 
shows  alternately  broad  and  narrow  strise,  the  former  being 
dim,  while  the  latter  is  bright  in  appearance.  With  a  stronger 
power  both  the  broad  and  narrow  strise  are  seen  to  be  trans- 
versely striated. 

Muscular  Tissues  of  the  Cheeks. — The  muscles  entering 
into  the  construction  of  the  lateral  walls  of  the  mouth  have 
already  been  described  in  part  i,  page  24,  giving  the  relations 
existing  between  the  individual  muscles,  together  with  the  gen- 
eral disposition  of  the  various  fasciculi.  Histologically  con- 
sidered, these  muscles  partake  of  all  the  characteristics  of 
striated  or  voluntary  muscular  tissue.  In  the  body  of  the  buc- 
cinator and  masseter  muscles  the  fibers  are  cylindric  and  have 
definitely  pointed  or  rounded  ends.  Near  their  termini,  par- 
ticularly in  the  latter  muscle,  the  inner  extremities  of  the  ter- 
minal fibers  are  pointed,  while  the  outer  ends,  or  those  by  which 
the  attachment  is  formed,  are  broad  and  rather  flat. 

Muscular  Tissues  of  the  Soft  Palate. — The  disposition  ot 
the  striated  muscular  tissue  of  the  soft  palate  is  extremely  com- 
plicated. The  azygos  uvulee,  the  only  true  longitudinal  muscle 
in  the  soft  palate,  has  its  origin  from  aponeurosis  of  the  soft 
palate  and  from  the  nasal  spine  of  the  palate-bone,  the  fibers 
passing  backward  upon  either  side  of  the  median  line.  This  is 
a  double  muscle,  and  near  its  point  of  origin  the  two  portions 
are  distinct  and  separated  by  a  definite  space,  but  upon  reaching 
the  base  of  the  uvula  they  become  closely  associated.  The 
fasciculi  do  not  continue  to  the  apex  of  the  uvula,  but  imme- 
diately beyond  the  center  of  its  length  are  thrown  out  fan-like 
toward  the  sides,  terminating  in  a  manner  similar  to  the  fibers 
of  the  lips.  In  passing  from  before  backward  a  number  of 
small  fasciculi  are  given  off  which  reach  out  laterally  and 
traverse  the  glandular  lobes,  completely  surrounding  them, 
after  which   they   again    return    to   the   principal   fibers   at   the 


MUSCULAR   TISSUES   OF   THE   TONGUE.  387 

median  line.  The  palatopharyngeus  muscle  is  divisible  into 
two  parts,  the  upper  extremities  of  which  lie  partly  in  front  and 
partly  behind  the  levator  muscles.  The  greater  number  of  the 
fibers  of  one  set,  situated  in  front  of  the  levators,  iorm  a  curved, 
flattened  aponeurosis.  The  fibrous  border  of  the  hard  palate 
serves  as  an  attachment  for  the  convex  border  of  this  portion, 
while  the  other  border,  which  is  concave,  is  directed  toward  the 
arch  of  the  levators.  The  fibers  of  the  palatopharyngeus, 
situated  behind  the  levators,  form  a  number  of  loose  fasciculi 
interspersed  by  fat-cells.  In  passing  toward  the  free  border 
of  the  soft  palate  the  fibers  become  much  more  delicate, 
and,  separating,  some  course  in  front  and  others  behind  this 
muscle.  In  this  location  the  fibers  become  closely  associated 
with  the  glands,  and  either  end  here  or  are  continued  to  the 
submucosa,  or  even  to  the  membrana  propria  of  the  mucous 
membrane.  The  fibers  of  the  palatopharyngeus  unite  with  the 
fibers  of  the  levators,  and  an  arch-like  fasciculus  is  formed  by 
this  union  which,  subdividing,  passes  in  front  of  the  azygos 
uvulae  to  the  opposite  side.  All  of  these  fibers  run  outward 
and  downward,  and  unite  with  the  extremities  of  the  other 
palatal  muscles,  the  fibers  of  which  are  somewhat  more  regu- 
larly distributed.  Like  the  muscles  of  the  lips  and  cheeks,  the 
several  fasciculi  of  the  palatal  muscles  form  a  delicate  plexus, 
and  a  quantity  of  fatty  tissue  is  found  between  the  various 
fasciculi. 

Muscular  Tissues  of  the  Tongue. — The  tongue  is  divided 
into  two  equal  lateral  portions  by  a  median  septum — the  septum 
linguae.  This  central  septum,  composed  of  a  vertical  layer  of 
compact,  fibrous,  connective  tissue,  extends  the  entire  length 
and  depth  of  the  lingual  median  line.  Beginning  at  the  hyoid 
bone,  it  gradually  increases  in  prominence  until  the  middle  of 
the  organ  is  reached,  beyond  which  point  it  becomes  less  pro- 
nounced and  finally  disappears  near  the  tip.  The  bundles  of 
the  muscular  tissues  are  arranged  longitudinally,  transversely, 
and  vertically.  The  former  lie  immediately  beneath  the  mucous 
membrane,  including  the  superior  lingualis  above  and  the  in- 
ferior lingualis  below,  together  with  the  greater  part  of  the 
styloglossus.     The  superior  lingualis  extends  from  the  base  to 


388  HISTOLOGY. 

the  tip  of  the  organ,  and  by  short  fasciculi  its  fibers  are  attached 
to  the  overlying  tissues.  The  fibers  of  this  muscle  are  placed 
between  the  hyo-  and  styloglossi  muscles  of  the  opposite  side, 
both  of  which  overlap  the  fibers  of  the  llngualls  near  the  base 
of  the  tongue.  The  Inferior  llngualls  also  gives  off  several 
small  fasciculi  and  fibers  to  the  mucous  membrane  beneath,  and 
is  composed  of  two  bands  which  reach  from  the  base  to  the 
apex,  each  being  placed  between  the  hyoglossus  and  genlo- 
hyoglossus  muscles.  The  transverse  fibers,  which  are  placed 
between  the  superior  and  Inferior  llngualls  muscles,  originate 
from  the  septum  linguae,  and  form  the  bulk  of  the  organ.  From 
their  point  of  origin  these  fibers  course  outward  and  upward  to 
the  sides  of  the  tongue.  Those  fibers  which  are  vertically  dis- 
posed decussate  with  the  transverse  fibers,  and  pass  from  the 
dorsum  toward  the  under  surface  of  the  tongue,  the  fibers 
curving  gracefully  with  their  concavity  directed  toward  the 
under  surface.  In  most  instances  the  ascending  vertical  fibers, 
as  well  as  the  transverse  fasciculi,  pass  between  those  longitu- 
dinally disposed  and  connect  with  the  submucosa. 


CHAPTER  V. 

TISSUES    OF    THE    TEETH-ENAMEL;      DENTIN;      CEMENTUM  ;     THE 
TOOTH-PULP.— THE  ALVEOLODENTAL  MEMBRANE. 

TISSUES   OF  THE   TEETH. 

Knamel. — The   enamel,    which    forms  a    cap-Hke    covering 
of  varying  thickness  over  the  entire  crown  of  the  tooth,  is  a 


Fig.  253. — Section  of  Enamkl  from  Human  Tooth  (Specimen  by  J.  Howard  Mum- 
mery).     X  350- — {After  Williavis.) 

vitreous,  hyaUne  substance,  containing  but  little,  if  any,  organic 
matter.     The  thickness  of  the  enamel  cap  appears  to  be  strongly 

3S9 


HISTOLOGY. 


influenced  by  the  function  of  thie  different  parts  of  the  tooth- 
crown,  being-  thickest  over  the  cutting-edges  of  the  anterior 
teeth,  while  in  tlie  cuspidate  teeth,  the  entire  occlusal  surface  is 
provided  with  the  thickest  enamel  layer.  It  is  about  evenly  dis- 
tributed over  the  lateral  walls  of  the  crown,  but  as  the  cervical 
line  is  approached, its  thickness  is  gradually  diminished  (Fig.  254). 
Chemically,  enamel  is  composed  of  the  salts  of  lime,  calcium 
phosphate  predominating.  Calcium  carbonate,  magnesium  phos- 
phate, and  calcium  fluorid  are  present  in  smaller  quantities. 
The  proportionate  quantity  of  lime-salts  in  enamel  is  not  fixed, 
a  slight  variation  in  density  occurring  in  the  enamel  of  different 
individuals.  These  essential  differences  are  regulated  by  the 
proportionate  quantity  of  calcium  phosphate  and  carbonate — 
a  greater  amount  of  the  former  being  productive  of  additional 
hardness,  while  an  increase  in  the  latter  beyond  the  minimum 
amount  decreases  this  quality.  As  a  general  rule,  the  teeth  of 
males  contain  a  greater  amount  of  calcium  phosphate  than  the 
teeth  of  females,  as  shown  by  the  following  analysis  by  von  Bibra: 


Calcium  phosphate  and  fluorid, 

Calcium  carbonate. 

Magnesium  phosphate, 

Other  salts, 

Cartilage, 

Fat, 

Total  organic. 
Total  inorganic, 


Man. 

89.82 

4-37 

1-54 

.88 

3-39 


3'59 
)6.4i 


Woman. 

81.63 

8.88 

3-55 

•97 

5-97 

a  trace. 

5-97 
94-03 


In  general  structure  enamel  is  composed  of  numerous  hex- 
agonal prisms,  with  a  common  direction  at  right  angles  to  the 
long  axis  of  the  tooth.  These  prisms  are  known  as  enamel 
■prisms,  enamel  fibe7-s,  or  enainel  rods.  While  the  general  direc- 
tion of  the  fibers  is,  as  previously  stated,  nearly  at  right  angles 
to  the  body  of  the  tooth,  they  do  not  pursue  a  perfectly  straight 
course  in  passing  from  the  dentin  to  the  surface,  but  are  dis- 
posed in  a  tortuous  or  wave-like  manner.  The  enamel  prisms 
may  be  said  to  sit  on  end  against  the  surface  of  the  dentin, 
minute  depressions  in  the  latter  receiving  the  extremities  of  the 
rods.     The  direction  of  the  enamel  prisms,  as  compared  to  the 


ENAMEL.  391 

body  of  the  tooth-crown,  varies  according  to  the  part  of  the 
crown  which  they  occupy.  Taking  the  entire  crown  of  the  tooth, 
they  radiate  in  such  a  manner  from  the  surface  of  the  dentin 
that  at  the  cutting-edge  or  occlusal  surface  of  the  tooth  they  are 
more  or  less  vertical,  while  over  the  lateral  surfaces  they  tend 
to  the  horizontal  direction.  An  examination  of  the  prisms 
when  isolated  and  decalcified  exhibits  numerous  evenly  dis- 
tributed varicosities,  producing  a  transversely  striated  appear- 
ance to  the  rods.     Tomes  has  pointed  out  that  the  enamel  rods 


Fig.  254. — Enamel  and  Dentin  from  Human  Tooth,  showing  Gradual  Reduction 
IN  THE  Thickness  of  the  Former  as  the  Cervix  is  Approached.     V  lO- 


are  variously  disposed  in  that  portion  of  the  enamel  most  closely 
associated  with  the  dentin.  On  the  cusps  of  the  teeth  they  are 
twisted  and  curved  in  various  directions,  while  near  die  surface 
on  the  incisors  they  are  uniform  and  straight.  In  general,  the 
enamel  rods,  which  begin  on  the  surface  of  the  dentin,  are  con- 
tinuous through  the  entire  thickness  of  the  enamel.  In  passing 
from  the  interior  to  the  exterior,  the  individual  rod,  to  occupy  a 
proportionate  space  in  all  parts,  would  have  to  increase  in  diam- 
eter;  but  this  it  does  not  do.      In  consequence  of  this  arrange- 


392  HISTOLOGY. 

ment  there  exist  numerous  supplemental  or  peripheral  rods, 
which  extend  but  a  short  distance  from  the  surface,  filling  in  the 
interprismatic  spaces  formed  by  the  longer  rods.  With  the 
exception  of  the  faint  transverse  striations,  the  enamel  prisms 
appear  to  be  structureless.  A  variety  of  opinions  have  been 
expressed  in  regard  to  the  cause  for  the  striated  appearance  of 
the  enamel  rods.  It  was  claimed  by  Hertz  to  be  attributable 
to  a  temporary  arrest  of  calcification,  but  more  recent  investi- 
gation has  shown  the  cause  to  be  the  presence  of  varicosities 


Fig.  255. — Comparison  in  the  Appearance  of  the  Enamel  and  Dentin  under  Low 
Power  of  the  Microscope.     X  4°- 


in  the  individual  fibers,  precisely  as  the  varicosities  in  the 
muscular  fibers  produce  the  striated  appearance  in  that  tissue. 
Bodecker  asserts  that  fully  developed  normal  enamel  is  non- 
striated,  and  Von  Ebner  practically  makes  the  same  statement, 
claiming  that  they  are  due  to  the  preparation  of  the  specimen, 
which  usually  being  mounted  in  Canada  balsam,  suffers  suffi- 
ciently from  the  slight  acid  reaction  to  produce  the  striated 
appearance.  These  statements  Williams  emphatically  denies, 
saying  that,  while  in   some  specimens  the  varicosities  are  ap- 


parent  in  some  parts,  they  are  decided  in  others.  If  this  be 
accepted,  it  would  seem  to  entirely  overthrow  the  theory  of  Von 
Ebner  in  regard  to  the  action  of  the  acid,  which  would  be  dis- 
tributed to  all  parts  alike. 

According  to  Williams,  the  varicosities  ot  one  enamel  prism 
are  opposite  those  of  the  adjoining  prisms,  and  by  the  coming 
together  of  the  varicosities  the  prisms  become  united  by  means 
of  processes  which  they  send  out.  In  like  manner  the  varicosi- 
ties  upon    the   same   rod   are   connected  by  processes  running 


Fig.  256. — Human  Enamel.     Transverse  Ground  Sections. — {After  Gysi.) 

parallel  with  the  prism.  According  to  Von  Ebner,  enamel  is 
traversed  by  numerous  minute  canals,  and  Heitzmann  claims 
to  have  found  organic  fibers  in  its  substance.  Williams,  while 
admitting  the  enamel  structure  to  be  far  more  complex  than 
past  research  has  shown,  appears  to  have  fully  demonstrated 
that  neither  canals  nor  organic  fibers  are  present — in  fact,  he 
denies  the  presence  of  the  least  trace  of  organic  matter  in  this 
structure.  The  interprismatic  matrix,  heretofore  considered  by 
most  authorities  to  be  an   organic  structure,   now  appears,  by 


394  HISTOLOGY. 

the  thorough  methods  employed  by  the  last-named  gentleman, 
as  a  transparent,  inorganic  substance.  By  numerous  experi- 
ments he  was  enabled  to  secure  a  specimen  in  which  the  inter- 
prismatic  spaces  of  one  layer  were  not  backed  up  by  the  rods 
of  another  layer.  In  some  instances  the  specimen  showed  the 
rods  well  separated  and  the  interspace  closed  by  a  perfectly 
transparent  substance,  in  the  interior  of  which  might  be  seen 
connecting  processes  passing  from  one  rod  to  another.  That 
the  enamel  is  practically  of  inorganic  material,  and  therefore 
not  capable  of  transmitting  or  receiving  sensations,  may  be 
demonstrated  by  the  simple  experiment  of  immersing  a  thin 
section  of  this  tissue  in  a  weak  solution  of  chromic  acid,  the 
result  of  which  will  be  a  speedy  separation  of  the  enamel 
prisms  which  have  been  liberated  by  the  destruction  of  the 
interprismatic  substance.  What  does  this  signify?  Chromic 
acid  is  one  of  the  best  preservatives  of  organic  tissue  known, 
and  if  the  cementing  substances  in  enamel  were  organic  or  even 
partly  so,  the  prisms  by  this  test  would  not  be  freed,  but  instead 
would  become  more  firmly  cemented  together.  Therefore  we 
infer  from  this  that  the  interprismatic  substance  is  even  less 
highly  organic  than  the  prisms  themselves,  these  not  being  acted 
upon  until  after  the  material  which  holds  them  together.  In  ad- 
dition to  the  striated  appearance  formed  by  the  varicosities  of 
the  individual  prisms  of  fully  developed  enamel,  other  structures 
of  a  different  character,  and  upon  a  much  larger  scale,  are 
present  and  known  as  the  "brown  striae  of  Retzius"  (Fig.  257). 
These  markings,  readily  seen  with  a  low  power,  are  of  a 
brownish  color,  and  run  nearly  parallel  with  the  surface  of  the 
dentin  or  enamel.  Those  strise  nearest  the  surface  of  the 
dentin  are  inclined  to  follow  the  contour  of  that  structure,  ex- 
tending in  many  instances  the  entire  length  of  the  crown.  The 
lines  nearest  the  surface  are  longest  in  the  region  of  the  cutting- 
edge,  or  occlusal  surface  of  the  crown,  becoming  shorter  as  the 
neck  of  the  tooth  is  approached,  being  directed  at  an  acute 
angle  to  the  surface  of  the  dentin  at  that  point.  A  number  of 
theories  are  advanced  to  account  for  the  presence  of  the 
"brown  strise  of  Retzius."  Tomes  suggests  that,  coinciding  as 
they  do  with  the  outer   surface  of   what  was   at  one  time  the 


ENAMEL.  395 

primitive  enamel  cap,  they  might  be  considered  as  in  a  measure 
outhning-  the  stratifications  of  the  primary  deposit.  Another 
theory,  but  one  seemingly  without  foundation,  is  to  the  effect 
that  the  strife  are  produced  by  an  arrest  in  the  calcifying 
process  ;  while  a  third  theory  attributes  the  cause  to  a  variation 
in  the  character  of  the  nourishment  taken  by  the  mother  during 
pregnancy.  The  acceptance  of  this  latter  theory  would  seem  to 
indicate  a  set  diet  for  all  mothers  and  at  stated  intervals,  the 


Fig.  257. — Thick  Section  of  Enamel  oi'  Human  Tooth,  showing  Brown  Stri/E  of 
Retzius.     X  40. 

striae  always  being  present  and  somewhat  regularly  distributed 
throughout  the  tissue. 

Still  another  set  of  lines  or  markings  are  to  be  observed 
in  the  substance  of  sections  of  enamel,  these  being  known  as 
the  "lines  of  Schreger."  Figure  258  shows  these  lines  as  they 
appear  in  the  enamel  by  reflected  light,  the  same  being  quite 
invisible  by  transmitted  light.  The  presence  of  these  lines  is 
due  to  the  various  directions  assumed  by  the  contiguous  groups 
of  enamel  rods.  Beginning  at  the  surface  of  the  dentin  they 
are  well  defined,  but  gradual!}^   become  less  marked  as  the  ex- 


396 


HISTOLOGY. 


Fig.  258. — Enamel  and  Dentin,  Human  Tooth. — [Afiei-  Gysi. 


Oral  Epithelium 
Heaped  up 
over  Band 


Connective  Tissue 


Fig.   259. — Vertical  Section  through  Jaw  ni    Human  Embryo.     Formation  of 
Tooth-band,  akout  .Sixtieth  Day.     /.   300. 


terior  of  the  enamel  is  approached.  At  the  line  of  union  be- 
tween the  enamel  and  dentin  irregularly  formed  cavities  are 
occasionally  observed,  into  which  the  dentinal  tubules  may  ex- 
tend, and  in  rare  instances  individual  tubules  may  pass  beyond 
the  boundary-line  of  the  dentin  and  enter  the  enamel,  but  in  all 
probability  both  of  these  conditions  are  pathologic.  Such  a 
state  of  affairs  could  hardly  be  considered  normal  when  we 
take  into  consideration  that  the  dentin  and  enamel  calcify  in 
opposite  directions,  and  that  the  outer  wall  of  the  former  is 
completed  before  enamel  calcification  begins. 


Ep'Uielii 


^^^'WTW^^. 


Infant 
Layer 


Connec- 
tive 
Tissue 


Fig.   260.— Vertical  Section,  Tooth-band,  Human  Embryo.     Tenth  Week.     X  3°°- 


Development  of  Enamel. — Preparations  for  the  development 
of  the  enamel  begin  toward  the  close  of  the  second  fetal 
month,  appearing  first  as  a  multiplication  of  the  primitive  epi- 
thelial cells  in  the  form  of  a  continuous  linear  projection  extend- 
ing somewhat  obliquely  into  the  subjacent  connective  tissue. 
From  this  crest,  or  tooth-band,  the  germs  for  the  future  enamel 
organs  are  given  off.  These  primary  dental  bulbs,  as  they  are 
called,  number  one  for  each  tooth  to  be  generated,  and  coinci- 
dendy  with  their  appearance  an  aggregation  of  closely  associated 


398  HISTOLOGY. 

connective-tissue  cells  make  their  appearance  in  the  surrounding 
submucous  tissue.  This  papilla-like  specialization  of  the  submu- 
cous tissue  is  the  primitive  dentin  germ,  or  dentin  papilla.  It 
will  thus  be  observed  that  the  enamel  is  a  product  of  the  surface 
epithelium,  ecderritic,  while  the  dentin  is  generated  from  the 
connective  tissues,  endermic.  Soon  after  the  appearance  of  this 
club-shaped  thickening,  or  tooth-bulb,  by  further  differentiation 
its-  form  becomes  bell-shaped,  with  the  concavit}'  directed  toward 
the  surface.  The  dentin  papilla  gradually  pushes  into  the  con- 
cavity of  the  forming  enamel  organ,  and  at  a  later  period  the 

Dental  Ridge 


Connective  Tissue 


Fig.  261.— Same  as  Figure  260.     About  Twelfth  Week.     X  200. 


odontoblastic  cells  are  generated  about  the  periphery  of  the 
papilla,  closely  followed  by  a  surface  calcification  of  the  dentin. 
Soon  after  the  forming  of  the  external  layers  of  dentin  the 
ameloblasts  or  enamel-forming  cells  become  active,  and  a 
deposition  of  enamel  prisms  takes  place  upon  the  exterior  of 
the  dentin  cap. 

Before  taking  up  the  subject  of  enamel  calcification,  brief  ref- 
erence will  be  made  to  the  further  development  of  the  enamel 
oro-an.  As  the  growth  of  this  organ  proceeds  there  is,  as  the 
result  of  a  rapid  proliferation  of  the  cellular  structure,  a  marked 


tendency  for  the  organ  to  become  separated  from  the  tooth-band. 
The  peripheral  cells  or  external  epitlieliiini  are  columnar  or 
prismatic,  and  remain  so,  while  those  in  the  center,  primarily 
polygonal,  soon  become  transformed  into  a  radiating  network 
or  stellate  reticulum.  The  appearance  of  a  stellate  reticulum  is 
first  observed  to  take  place  in  the  cells  occupying  the  central 
portion  of  the  enamel-organ,  this  cellular  transformation  pro- 
gressing from  the  center  outward,  but  ceasing  before  reaching 
the  columnar    surface  cells    contiguous    to  the  dentin    papilla. 


:-iWi 


Epillieliumof 
Upper  Jaw 


Neck  01  Cord 


J      1  iiamel  Org 
-J    Dentin  Papilla 


'^-: 


Fig.  262. — Vertical  Section  through  TiH.rn.iuM].  Cord,  Dentin  Papilla,  and 
Enamel  Organ,  about  Fifteenth  Week.     X  'S°- 

Between  the  enamel  cells  and  the  stellate  reticulum  is  a  layer 
of  unaltered  cells — the  stratum  intermedut-in.  In  the  earlier 
stages  of  the  development  of  the  enamel  organ  the  peripheral 
cells  are  alike,  being  columnar  or  prismatic,  but  almost  coinci- 
dent with  the  appearance  of  the  dentin  papilla,  the  cells  most 
closely  related  to  it  are  observed  to  become  elongated,  and 
form  the  internal  epithelium  of  the  enamel  organ.  As  the  cells 
forming  this  internal  epithelium  become  elongated,  their  nuclei, 
instead  of  occupying  the  center  of  the  protoplasmic  body,  are 
carried  to   their  extremities.     It  will  thus  be  seen  that  the  com- 


400  HISTOLOGY. 

pleted  enamel  organ  consists  of  four  divisions  or  layers  of  cells. 
Beginning  with  its  convex  surface  is  an  external  epithelium  or 
outer  tunic,  successively  followed,  in  passing  toward  the  dentin 
papilla,  by  a  stellate  reticulum,  stratum  intermedium,  and  an 
internal  epithelium  or  inner  tunic.  As  the  growth  of  the  enamel 
organ  proceeds,  the  tooth-band  becomes  smaller  and  smaller  in 
size,  until  finally  a  complete  rupture  takes  place.     This  rupture. 


-Ti 


JD. 


—  a 


Fig.  263. 


-Section  of  a  Developing  Tooth  of  Lamb.     X  600. 
a,  Ameloblasts ;  D,  dentin  ;   P,  dental  pulp. 


however,  does  not  occur  until  the  enamel  organ  has  about  or 
fully  completed  its  development,  and,  after  remaining  so  long 
under  the  influence  of  the  oral  epithelium,  it  must  be  considered, 
as  before  stated,  an  epithelial  structure.  It  is  through  the 
agency  of  the  internal  epithelial  cells  of  the  enamel  organ,  the 
enamel  cells  or  ameloblasts,  that  calcification  of  the  enamel  takes 
place,  and  that  subject  will  next  be  considered. 


AineIiJicat2on{¥\g.  263). — Two  theories  are  advanced  in  regard 
to  the  calcification  of  the  enamel.  In  one  it  is  claimed  that  the 
ameloblasts  or  enamel  cells  become  directly  calcified  or  converted 
into  enamel ;  in  the  other  the  ameloblasts  are  simply  considered 
as  controlling  agents,  by  secreting  or  depositing  the  calcium  salts 
which  form  the  enamel  prisms.  In  the  latter  theory  it  is  gener- 
ally believed  that  the  enamel  is  secreted  or  shed  out  from  the 


~^ 

:gK„an,e, 
,•-§;      Rods 

''M 

jiW     Outer 

7/  Laverof 
■  ./      E.iamel 

im^s 

— ^ — 1.-1 —    Papillse 

200.  —  [After  Williams.) 


extremities  of  the  ameloblasts,  thus  being  productive  of  enamel 
fibers  corresponding  in  size  and  position  to  the  secreting  cells. 
By  the  direct  calcification  of  the  ameloblasts,  it  would  be  natural 
to  expect  the  process  to  begin  on  the  exterior  of  the  cell  and 
gradually  pass  into  its  interior,  the  central  portion  being  the  last 
to  calcify.  This  would  result  in  an  enamel  prism  corresponding 
in  size  and  form  to  the  generating  cell,  and  in  a  measure  this 
26 


402  HISTOLOGY. 

similarity  between  the  calcified  and  uncalcified  structure  does 
exist  and  is  one  of  the  potent  factors  in  the  recognition  of  this 
theory,  but  it  is  hardly  sufficiently  convincing  to  warrant  a  gen- 
eral acceptance  of  the  beliel. 

By  examining  figure  263  the  importance  of  the  secretory 
theory  is  favored.  Here  we  note  that  the  early  formed  enamel  at 
A  records  what  appears  to  be  the  definite  action  of  the  ame- 
loblasts  by  prolongations  of  partly  calcified  tissue  extending 
from  the  cells,  these  markings  corresponding  in  number  and 
location  to  the  cells  themselves.  Between  these  prolongations 
and  the  ameloblasts  are  many  highly  refractive  granular  bodies 
which  seem  to  be  the  actual  lime  deposit  shed  out  from 
the  free  extremities  of  the  cells,  from  which  they  pass  into  the 
substance  of  the  then  organic  matrix  beyond  at  A  and  form  the 
enamel  prisms.  Williams  takes  exception  to  this  latter  view, 
substantiating  his  opinion  by  stating  that  while  the  ameloblasts 
of  many  animals  are  similar  in  shape  and  arrangement,  the 
enamel  produced  from  these  similarly  arranged  cells  varies 
greatly  in  structure.  The  same  writer  also  states  that,  when 
such  a  similarity  of  arrangement  exists  between  the  amelo- 
blasts and  the  enamel  prisms,  it  occurs  near  the  commence- 
ment of  enamel  calcification,  and  that  at  a  later  period  the 
relative  position  of  the  ameloblasts  and  the  prisms  is  always 
in  longitudinal  section.  Dr.  Williams  calls  attention  to  the 
fact  of  the  enamel  prisms  or  rods  not  extending  through  the 
entire  distance  between  the  enamel  cells  and  the  calcified 
dentin  (Fig.  263).  That  part  of  the  structure  lying  between 
the  ameloblasts  and  the  extremities  of  the  enamel  rods  is 
made  up  of  a  double  set  of  fibers,  some  of  which  are  almost  at 
right  angles  with  the  long  axis  of  the  ameloblasts.  In  figure 
265  D  the  two  sets  of  fibers  previously  mentioned  are  found  to 
join  and  become  closely  interwoven. 

The  ameloblasts  are  connected  with  the  cells  of  the  stratum 
intermedium,  and  more  recent  investigation  goes  to  prove  that 
this  latter  structure  is  directly  interested  in  furnishing  to  the 
ameloblasts  the  proper  material  for  the  calcifying  process.  The 
stratum  intermedium  can  not,  however,  take  part  in  the  primary 
enamel    calcification,    as    this    process    commences    before    the 


stratum  intermedium  is  fully  developed.  This  being  the  case,  it 
is  generally  supposed  that  the  stellate  reticulum  furnishes  the 
material  for  the  upbuilding  of  the  first  enamel  prisms. 

Lying  between  the  free  extremities  of  the  ameloblasts  and 
the  enamel  in  the  course  of  formation  is  what  has  been  generally 
considered  as  a  structureless  basement  membrane  or  membrana 


of  Capillary  Loop 


Fig.  265. ^Section  of  Incisor  of  Rat,  showint.  Partial  Decalcification  of  Enamel 
X  600. — (After  Williains.) 

prcefonnativa.  The  existence,  exact  location  and  structure  ot 
this  membrane  has  been  and  still  remains  a  matter  of  conjec- 
ture. The  generally  accepted  theory  appears  to  be  that  given 
above,  but  Williams  refers  to  it  as  a  layer  of  newly  formed 
enamel,  and  does  not  consider  it  as  a  structureless  membrane. 
The  structure  is  to  be  observed  in  figure  265,  at  either  extremity 


404  HISTOLOGY. 

of  the  ameloblastic  cells,  this  writer  claiming  that  the  so-called 
structureless  membrane  is  present  at  both  of  these  points. 

As  to  the  formation  of  the  enamel  rods,  Dr.  Andrews,  in 
1894,  referring  to  the  presence  of  calcoglobulin  in  the  enamel 
cells  considered  these  refractive  bodies  as  calcospherites,  which, 
after  being  taken  up  by  the  ameloblasts,  were  excreted  by 
them,  and  after  coalescing  formed  globules  of  larger  size,  from 
which  the  rods  were  built  up.  Williams  partly  agrees  with  this 
statement,  but  he  is  of  the  opinion  that  the  calcospherites 
coalesce  while  in  the  ameloblasts,  forming  large,  spheric 
bodies,  but  that  the  deposit  of  this  substance  is  in  no  way  pro- 
ductive of  building  the  enamel  rods.  The  theory  of  Tomes  in 
regard  to  the  forming  of  enamel  rods  was  that  the  walls  of  the 
ameloblasts  themselves  became  calcified,  while  the  contents  of 
the  cells  also  became  solidified,  the  first  forming  the  interpris- 
matic  substance,  while  the  second  became  the  enamel  rod. 
Whatever  theory  be  accepted  as  to  the  formation  of  the  enamel 
rods  or  prisms,  there  appears  to  be  no  question  in  regard  to  the 
general  process  of  enamel  calcification.  In  the  first  place,  an 
organic  matrix  is  formed,  into  which  the  first-formed  layer  of 
enamel  is  deposited.  Gradually  the  organic  matter  disappears, 
leaving  behind  the  inorganic  elements  closely  resembling  in 
appearance  the  organic  matrix,  which  it  has  by  atomic  change 
supplanted.  The  question  of  an  organic  interprismatic  cement- 
substance  is  also  one  upon  which  various  writers  disagree. 
Klein  partly  believes  that  such  a  substance  does  exist,  basing  his 
opinion  upon  the  fact  that  the  ameloblastic  cells,  in  common 
with  all  epithelial  cells,  are  separated  from  one  another  by  a 
homogeneous  intercellular  substance,  and  that  a  certain  propor- 
tion of  this  organic  substance  must  remain  between  the  enamel 
prisms  after  calcification.  Dr.  Sudduth,  by  a  series  of  experi- 
ments made  some  years  ago,  appeared  at  that  time  to  have 
furnished  conclusive  proof  that  an  organic,  interprismatic 
cement-substance  does  not  exist  between  the  enamel  prisms  of 
fully  developed  enamel.  By  the  use  of  a  dilute  solution  ot 
chromic  acid,  the  action  of  which  is  the  preservation  of  organic 
substance,  the  prisms  were  liberated,  which  would  not  have  been 
the  result  had  they  been  cemented   by  an  organic  cement-sub- 


ENAMEL.  405 

Stance.  By  substituting  dilute  muriatic  acid,  the  action  of  which 
is  the  destruction  of  organized  tissue,  the  prisms  were  not  liber- 
ated, the  acid  acting  evenly  upon  the  whole  mass  of  enamel,  and 
finally  resulting  in  its  complete  destruction,  not  leaving  the  slight- 
est trace  of  an  organic  matrix  behind.  Dr.  Williams  claims  that 
this  transparent  cement-substance  is  formed  by  the  distribution 


O  liter  Amelo 
blastic  Mem- 
brane 


Flu.  266. — Section  of  Developing  Tooth  of  Embryo  Lamb.    X  i5°- — (^fter  Williams.) 


of  a  translucent  liquid  substance  about  the  previously  formed 
pattern  for  the  enamel  rods.  This  pattern,  generated  through 
the  activity  of  the  enamel  cells,  is  composed  of  a  translucent 
material  somewhat  more  solid  than  that  structure  which  sur- 
rounds it.  These  two  substances  calcify  together,  the  latter 
forming  the  enamel  prisms,  while  the  former  creates  the  cement 


4o6 


HISTOLOGY. 


or  interprismatic  substance.  There  is  no  better  method  by 
which  to  study  the  character  or  mode  of  development  of  a  grow- 
ing or  matured  tissue  than  by  an  artificial  disassociation  of  its 
component  parts.  In  enamel  it  matters  but  little  in  what  part 
of  the  tissue  or  at  what  period  of  its  growth  the  examination  be 
made  to  learn  of  the  action  of  the  decalcifying  agent ;  it  will 
never  be  found  to  take  place  in  a  manner  corresponding  to  the 


Fig.   267. 


direction  of  the  enamel  fibers,  but  decalcification  takes  place  in 
older  enamel  more  in  the  form  of  a  general  breaking  up  of  the 
structure,  as  shown  at  A  in  figure  267,  while  at  B,  which  repre- 
sents the  newly  formed  tissue,  the  action  is  one  which  appears 
to  indicate  a  breaking  up  of  the  interprismatic  substance,  favor- 
ing the  theory  of  secretory  amelification. 

This  distinction  between  the  enamel  cells  and  their  product 


ENAMEL. 


407 


possesses  none  of  the  characteristics  to  properly  classify  it  as 
structureless.  There  is  but  little  doubt  as  to  its  character,  being 
the  primary  product  of  the  ameloblasts,  while  the  corresponding 
zone  at  the  distal  end  of  the  enamel-forming  cells  results  from 
the  functional  activity  of  the  stratum  intermedium. 

Figure  268  shows  a  section  near  the  point  of  the  cusp  of  a 


developing  molar  and  exhibits  a  portion  of  the  enamel  organ  at 
a  time  immediately  prior  to  the  beginning  of  enamel  calcifica- 
tion. A  is  the  uncalcified  dentin,  B  the  ameloblasts,  now 
closely  associated,  and  a  very  regular  layer  of  elongated 
cells,  and  behind  these  another  layer  of  cells,  which  un- 
doubtedly serve  as  feeders  to  the  ameloblasts,  the  stratum  inter- 


4oS  HISTOLOGY. 

medium.  This  section  is  especially  valuable  in  that  it  shows  a 
number  of  capillaries  distributed  through  the  body  of  the  stellate 
reticulum  and  actually  penetrating  the  stratum  intermedium,  as 
seen  at  B. 

When  viewed  with  a  low  power,  these  minute  blood-vessels 
appear  to  form  a  complete  network,  and  in  the  district  between 
the  cusps  pervade  the  entire  structure,  from  the  stratum  inter- 
medium on  one  side  to  the  same  cells  on  the  other.  The 
appearance  of  this  animated  vascular  supply  to  the  enamel  is 
coincident  with  the  process  of  calcification,  for  during  the  early 
life  of  the  tooth-germ  it  is  non-vascular.  The  growth  of 
enamel,  strata  upon  strata,  from  Avithin  outward  is  therefore  by 
the  direct  calcification  of  the  enamel  cells  or  ameloblasts,  and 
while  this  is  going  on,  and  as  long  as  the  crown  of  the  tooth  is 
incased  in  its  epithelial  cap,  the  enamel  organ,  the  growth  of  the 
tissue  is  stimulated  through  the  blood-vessels  everywhere  pres- 
ent in  the  stellate  reticulum.  The  presence  of  this  specialized 
blood  supply  to  the  central  portion  of  the  enamel  organ  was 
for  a  long  time  doubted,  but  at  present  it  can  be  readily  ob- 
served (Fig.  267).  As  soon,  however,  as  the  tooth  passes 
through  the  surface  tissue,  carrying  with  it  the  external  epithe- 
lium of  the  enamel  organ  as  the  enamel  cuticle,  the  possibility 
of  nourishment  has  been  cut  off,  and  after  a  little  time  it  be- 
comes a  petrified  dental  epithelium,  no  longer  nourished  and 
absolutely  non-vital. 

Dentin. — This  tissue,  which  constitutes  the  principal  bulk 
of  the  hard  part  of  the  tooth,  forms  a  complete  cap-like  in- 
vestment over  the  pulp,  from  which  it  is  generated.  It  is 
white  or  slightly  yellowish-white  in  color,  somewhat  elastic,  and 
a  trifle  harder  than  bone,  which  it  resembles  in  many  of  its 
characteristics.  In  a  perfectly  developed  tooth  no  part  of  the 
dentin  appears  upon  the  surface,  that  part  Avithin  the  crown 
being  covered  by  the  enamel,  while  that  of  the  root  is  inclosed 
by  the  cementum.  While  the  thickness  of  the  dentin  varies 
somewhat  over  the  different  parts  of  the  tooth,  there  is  a  de- 
cided disposition  to  an  equal  distribution  in  every  direction. 
Dentin,  unlike  enamel,  consists  of  an  organic  matrix — a  reticular 
tissue  of  fine  fibrils  richly  impregnated  with  the  salts  of  calcium. 


in  this  resembling  the  matrix  of  bone.  Traversing  the  matrix 
are  long,  fine  canals  or  tubes  (Fig.  270), — the  dentinal  tiihdes, — 
which  pass  from  the  margins  of  the  pulp  toward  the  surface. 
Immediately  surrounding  the  dentinal  tubules  the  matrix  is 
especially  dense,  forming  a  lining  or  sheath  to  the  tubes,  known 
as  the  dentinal  sheaths.  Occupying  the  lumen  of  the  dentinal 
tubules  are  solid  elastic  fibers — the  deiitinal  fibers.  Dentin, 
therefore,  presents  for  examination,  first,  the  matrix  ;  second,  the 
dentinal  tubules  ;  third,  the  dentinal  sheaths  ;  ,and  fourth,  the 
dentinal  fibers. 


-Section  through  Crown  of  Human  Cuspid. 


The  Matnx. — As  previously  stated,  the  matrix  is  composed 
of  organic  and  inorganic  substances,  but  the  proportionate 
quantity  of  organic  and  inorganic  constituents  Is  so  variable 
that  it  is  impossible  to  furnish  a  definite  chemic  analysis.  The 
relative  quantity  of  organic  and  inorganic  matter  is  not  only 
variable  in  the  teeth  of  different  individuals,  but  is  continually 
changing  in  the  teeth  of  the  same  individual,  the  former  being- 
present  in  larger  quantities  during  youth  and  gradually  dimin- 
ishing  as  age  advances.     From   an    examination   of  perfectly 


4IO  HISTOLOGY. 

dried    dentin,    the    following    approximate    analysis    has    been 
obtained : 

Organic  matter  (tooth-cartilage), 27.61 

Fat, 0.40 

Calcium  phosphate  and  fluorid, 66.72 

Calcium  carbonate, 3.36 

Magnesium  phosphate, 1.08 

Other  salts, 0.83 

The  organic  basis  of  the  matrix  appears  to  be  structureless 
and  transparent,  and,  although  closely  resembling  the  matrix  of 


Fig,  270. — Section  through  Root  of  Human  Incisor,  showing  many  Dentin 
Tubules  in  Transverse  Section.     X  200. 

bone,  is  not  identical  with  it.  While  the  matrix  is  usually 
structureless,  there  are  instances  in  which  the  presence  at  one 
time  of  connective-tissue  fibers  is  indicated. 

T/ie  Dentinal  Tzibiiles  (Figs.  271,  272,  273). — Beginning  by  a 
free  opening  about  the  walls  of  the  pulp-cavity,  the  dentinal 
tubules  permeate  the  matrix  in  all  directions.  The  tubules  are 
generally  disposed  in  a  direction  perpendicular  to  the  surface, 
so  that  in  different  parts  of  the  tooth  they  radiate  in  various 
directions.     Beginning  upon  the  surface  of  the  pulp-cavity,  at 


which  point  they  are  of  greatest  diameter,  they  pass  more  or 
less  in  a  spiral  manner  toward  the  surface  (Fig.  273),  before 
reaching  which  they  become  gradually  reduced  in  size,  as  a  result 
of  the  numerous  branches  which  they  give  off  (Fig.  272).  The 
branches  given  off  from  the  main  tubes  are  quite  variable  in  size, 
and  anastomose  with  one  another  or  with  the  branches  from  other 
tubules.  In  the  region  of  the  pulp  the  tubules  are  so  closely 
associated  that  but  little  space  is  provided  for  the  intertubular 
substance  or  matrix  ;  but  as  the  surface  is  approached  they 
become  more  widely  separated,  and,  in  consequence,  the  matrix 
substance  is  present  in  greater  abundance.      While  the  general 


Dentinal  Tubul. 


Fig.  271. — Dentin  and  Cementum  from  Root  of  Human  Molar. — (^After  Cysi.) 

direction  of  the  tubes  is  perpendicular,  they  do  not  pursue 
a  direct  course,  but  are  more  or  less  curved  as  they  pass 
from  within  outward.  The  curvature  of  the  tubuli  may  be 
divided  into  two  classes, — long  curves  and  short  curves, — 
usually  referred  to  as  the  primary  and  second  curvatures  of 
the  dentinal  tubules.  The  primary  curvatures  are  few  in  number 
and  are  most  prominent  in  the  crown,  while  the  secondary  curva- 
tures, principally  found  in  the  roots,  are  smaller  and  more 
numerous.  The  branches  from  the  main  tubes  terminate  in 
various  ways,  either  by  anastomosis,  by  gradually  fading  out  into 
hair-like  terminals,  or  by  ending  in  hooks  and  loops.     In  rare 


412  HISTOLOGY. 

instances  they  are  said  to  enter  the  substance  of  the  enamel  or 
cementum,  but  it  is  doubtful  if  they  do  so  normally.  The 
branches  from  a  main  tube  are  usually  two  in  number,  the  latter 
being  almost  equal  in  diameter  to  the  former,  and  from  this  first 
set  of  branches  a  number  of  minute  branches  are  given  off 
almost  at  right  angles.  In  the  crown  this  latter  class  of  tubules 
are  seldom  observed,  excepting  near  the  enamel  margin,  but 
in  the  root  they  are  everywhere  noticed.  Small  varicosities  are 
frequently  present,  but  not  in  sufficient  numbers  to  produce  a 
striated  appearance  on  the  surface  of  the  dentin. 


Fig.  272. — Longitudinal  Section  through  Root  of  Human  Molar.     Branching 
OF  THE  Dentinal  Tubules.     X  200. 

The  Dentinal  Sheaths. — While  the  dentinal  tubules  ramify 
through  the  matrix  in  the  form  of  well-defined  channels,  the  walls 
of  the  channels  are  not  formed  by  the  matrix,  but  by  an  indestruc- 
tible substance  the  exact  character  of  which  is  not  fully  under- 
stood. The  walls  of  the  tubes,  or  the  dentinal  sheaths,  as 
they  are  termed,  are  believed  by  some  histologists  to  be  cal- 
cified, while  others,  though  acknowledging  their  apparent  inde- 
structibility, are  doubtful  as  to  the  correctness  of  this  theory. 
Neumann  being  the  first  to  accurately  describe  the  walls  of 
the  tubules,  they  have  become  known  as  "  Neumann's  sheaths." 


The  existence  of  tlie  dentinal  sheaths  may  best  be  dem- 
onstrated by  subjecting  the  tissue  to  the  action  of  strong  acid 
for  a  sufficient  time  to  destroy  the  intervening  matrix,  which 
process  usually  requires  several  days.  The  fibrous  mass  re- 
maining will  be  found  to  contain  a  collection  of  tubes,  which, 
however,  by  careful  examination,  are  found  not  to  be  the 
dentinal  tubules  themselves,  but  the  walls  of  these  canals. 
Magitot  and  Sudduth  deny  the  existence  of  a  wall  to  the 
dentin   tubes.      Tomes,  while    inclined    to    the    belief   that    the 


Fig.  273. — Transverse  Section  through   R^hji   ..1    Human  Molar,  showing  the 
Curvature  ok  the  Dentin  Tubules  about  the  Pulp-canal.     X  4°- 


tubes  are  provided  with  definite  walls,  suggests  that  they  may 
have  been  produced  artificially  during  the  preparation  of  the 
specimen,  and  that  they  are  only  brought  into  existence  by  the 
action  of  the  agents  used  for  this  purpose.  In  conclusion,  the 
same  writer  adds  that  that  part  of  the  matrix  immediately 
surrounding  the  fibril  differs  in  its  chemic  constituents  from  the 
body  of  the  matrix. 

The  Dentinal  Fibers. — Occupying  the  lumen  of  each  dentin 
tube  is  a  soft,  elastic  fiber,  which  is  continuous  with  and  has  its 
origin  from  the  odontoblastic  cells  upon   the  periphery  of  the 


414  HISTOLOGY. 

pulp.  The  existence  of  these  elongated  processes  of  the  odon- 
toblasts having  first  been  demonstrated  by  Tomes,  they  are 
otherwise  known  as  Tomes'  fibers.  By  means  of  these  fibers, 
which  not  only  fill  the  lumen  of  the  larger  tubes,  but  the 
minute  branches  as  well,  the  substance  of  the  dentin  is  both 
nourished  and  rendered  slightly  sensitive.  There  is  still  some 
doubt  as  to  the  real  nature  of  the  fibrils,  but,  if  they  are 
processes  from  the  odontoblasts,  it  would  appear  that  the 
substance  would  be  identical  with  that  of  the  cell-protoplasm. 
Bodecker    claims    that    thev    are    not    round    but    inclined    to 


Fig.  274. — Longitudinal  Section  through  Root  of  Human  Tooth,  showing 
Primary  Curvature  ok  Dentin  Tubules.     X  4°- 

angularity,  but  Tomes  infers  that  this  form  has  been  pro- 
duced by  the  action  of  some  reagent.  Klein  advances  the 
theory  that  the  odontoblasts  are  active  in  the  generation  of 
the  matrix  for  the  dentin  only,,  and  that  the  dentinal  fibrils 
are  not  processes  from  them,  but  originate  from  cells  inter- 
vening between  the  odontoblasts  and  connecting  with  the 
dentin  tubes.  It  has  never  been  fully  demonstrated  that 
true  nerve-fibers  enter  the  dentin  along  with  or  in  the  sub- 
stance of  the  dentinal  fibril,  but,  while  the  evidence  is  not  at 
present  forthcoming,  there  is  but  little  doubt  that  the  sensitive- 


ness  of  the  dentin  is  produced  by  the  presence  of  organized 
tissue  in  the  tubuH.  Some  contend  that  the  contents  of  the 
tubules  are  made  up  of,  first,  a  creative  portion,  that  given  oft 
directly  from  the  odontoblasts  ;  second,  a  circulatory  portion,  a 
minute  vessel  traversing  each  tubule,  entering  either  by  the  side 
of  the  cell-bodies  or  passing  through  them,  and  that  the  nerve 
terminals  are  distributed  in  the  same  manner.  Others  say  that 
minute  nerve-filaments  from  the  pulp  pass  directly  through  the 
odontoblasts  and  are  continued  in  the  center  of  the  tubule  sur- 
rounded by  a  simple  connective  tissue,  the  cell  process,  and  that 


Cemen-                  . 

turn             .   ..■;   '^ 

:r^ .. 

•^^SS^-- 

'-'■,   '^ 

>0^^'^' '■■   ■ 

.^'. 


...,M*is*> -•**** 


-?;-<--, -^ 


Fig.  275. — Transverse   Section  through  the   Root  of  a  Human   Incisor,  showing 
THE  Dentin  Surrounded  ey  the  Cementum.     X  j^- 


in  this  way  sensations  are  conveyed.  It  is  now  generally  con- 
ceded that  dentin  is  a  highly  organized  connective  tissue  ;  that  it 
has  a  circulatory  system  and  is  endowed  with  sensation  to  a 
slight  degree  ;  that  these  conditions  are  brought  about  not  by 
actual  entrance  into  the  tubules  of  separate  vessels  and  nerve- 
filaments,  but  more  in  the  way  of  the  tubules  being  occupied  by 
a  general  connective-tissue  substance  resembling  in  all  essential 
features  the  pulp  itself,  being  the  semi-fluid  interfibrillar  ground- 
substance  of  the  pulp  ;  that  dendrites  of  sensory  neurons  every- 
where present  in  the  pulp,  after  losing  their  medullary  sheaths 


4l6 


HISTOLOGY. 


divide  into  fine  varicose  fibers  and  become  closely  associated 
with  the  peripheral  cells,  pass  between  these,  and  enter  the 
cone-shaped  openings  of  the  tubules  and  terminate  soon  after 
doing  so. 

While  the  microscope  reveals  in  some  instances  what  appear 
to  be  prolongations  from  the  dentinal  fibers  penetrating  the 
enamel,  or  between  its  prisms,  such  a  condition  is  improbable 
if  not  impossible.  If  this  arrangement  is  present  at  all,  it  is 
so  slieht  as  to   have  no   influence  whatever  over  the   enamel 


Fig.  276. — Tomes'  Granular  Layer.     X  4°- 


either  as  to  nourishment  or  sensation.  No  conclusions  can  be 
drawn  with  positive  certainty  from  sections,  since  the  slightest 
deviation  from  parallelism  in  the  surfaces  may  easily  produce 
deceptive  appearances.  It  is  just  as  common,  and  even  more 
so,  to  find  hair-like  lines  interwoven  and  running  parallel  with 
the  surface  of  the  dentin  immediately  between  this  tissue  and 
the  enamel,  as  it  is  to  see  slight  fibers  crossing  beyond  their 
boundary-line  to  penetrate  the  enamel.  The  most  likely  place 
of  all  to  find  such  a  condition  would  be  in  the  beginning  of  cal- 
cification, and  here  it  is  never  observed.     The  peripheral  pulp- 


cells,  usually  all  classed  as  odontoblasts,  are  never  found  outside 
their  own  territory,  the  dentinal  papilla  ;  but  their  location 
in  the  beginning  on  the  very  surface  of  the  papilla,  almost  in 
direct  contact  with  the  inner  tunic  of  the  enamel  organ,  would 
make  it  possible  for  their  processes,  when  appearing,  to  pene- 
trate between  the  cells  of  the  enamel  organ  if  they  were  grown 
out  from  the  body  of  the  cells  from  which  they  spring.  This, 
however,  they  do  not  do.  They  do  not  grow  out  from  the 
cell-body,  so  to  speak,  but  the  cell  recedes,  leaving  them  behind. 
By  this  arrangement  the  terminals  of  the  future  fibers  become 
definitely  established,  all  increase  in  length  taking  place  in  the 
opposite  direction,  toward  the  pulp.  While  the  active  enamel- 
forming  cells  are  present  some  little  time  prior  to  the  odonto- 
blasts, calcification  of  the  enamel  does  not  take  place  until  after 
a  definite  cap  of  dentin  has  been  formed,  imprisoned  in  which 
are  the  terminal  branches  of  the  fibers.  Therefore  the  fact  that 
this  cap  of  dentin  is  formed  first,  and  this  is  not  a  question  in 
dispute,  with  the  fibers  or  cell  processes  securely  encapsulated 
within  it,  would  seem  to  be  sufficient  evidence  to  qualify  the 
statement  that  the  dentinal  fibers  do  not  penetrate  the  enamel. 
The '  examination  ot  very  many  sections  of  young  growing 
teeth  exhibits  the  fact  that  the  early  formed  dentin  and  enamel 
will  separate  bodil)',  leaving  a  positive  clear  line  of  separation 
and  a  surface  absolutely  devoid  of  anything  resembling  the 
prolongation  of  the  fibers  extending  from  the  surface  of  the 
dentin. 

Interglobular  Spaces. — In  that  part  of  the  dentin  which 
immediately  underlies  the  cementum  numerous  intercommuni- 
cating, irregularly  branched  spaces  are  found.  These  are 
known  as  the  interglobular  spaces  (Fig.  276).  On  account  of 
the  granular  appearance  which  this  portion  of  the  dentin  ex- 
hibits under  low  magnifying  power.  Tomes  has  designated  it  as 
the  "granular  layer."  The  granular  layer  is  also  found  upon 
that  portion  of  the  dentin  which  underlies  the  enamel,  but  in  this 
region  it  is  far  less  marked.  Many  of  the  dentin  tubes  have 
their  endings  in  these  spaces.  While  the  interglobular  spaces 
are  most  numerous  near  the  peripheral  portion  of  the  dentin, 
they  are  by  no  means  confined  to  these  parts.  They  are  present 
27 


4l8  HISTOLOGY. 

in  all  parts  of  the  dentin,  but  not  so  closely  associated,  and  may 
be  observed,  when  a  dried  section  of  dentin  is  examined,  as 
spaces  with  irregular  outlines  and  sharp-pointed  processes  ex- 
tending in  various  directions  (Fig.  277).  The  term  "interglob- 
ular spaces"  becomes  partly  a  misnomer  when  the  so-called 
"spaces"  are  more  carefully  examined.  In  normal  dentin 
the  "spaces"  are  filled  with  a  soft,  living  plasma,  having  a 
structural  arrangement  similar  to  the  general  matrix  of  the 
dentin,  and  it  is  only  in  a  dried  specimen  that  a  true  space  is 
found  by  the  shrinking  or  shriveling  of  the  organic  contents. 
The  interglobular  spaces  forming  the  granular  layer,  which  are 


Fig.  277. — So-called  Interglobular  Stages   in   Dried  Section  of  Dentin.     X  '°o- 

much  more  numerous,  but  of  smaller  size,  than  those  found  in 
the  body  of  the  dentin,  are  also  filled  with  a  soft  living  plasma, 
and  they  communicate,  on  one  hand,  with  the  dentinal  fibers, 
and,  on  the  other,  with  the  lacunae  and  canaliculi  of  the  cem- 
entum.  According  to  Sudduth,  the  interglobular  spaces  (so 
called)  are  occupied  by  masses  of  calcoglobulin  which  have 
not  become  fully  calcified. 

Deiitmifi cation. — The  dentin  bulb,  or  papilla  from  which  the 
dentin  is  formed,  having  already  been  described  in  part  i, 
the  process  of  calcification  Avill  at  once  be  taken  up.  It  will 
be    recalled    that    calcification    of   the    dentin   does    not   beorin 


until  the  dentin  papilla  has  developed  to  the  form  and  size  of 
the  dentin  of  the  future  tooth-crown.  When  this  has  taken  place, 
there  is  generated  upon  the  surface  of  the  papilla  a  modified 
form  of  connective-tissue  cells  called  odontoblasts  (Fig.  278). 
These  cells,  which  are  arranged  in  a  single  row  upon  the  exterior 
of  the  papilla,  vary  in  form  according  to  their  activity.  When 
most  active,  they  are  broadest  at  the  extremity  directed  toward 
the  interior  of  the  papilla.  Proceeding  from  a  single  odonto- 
blast there  may  be  one  or  more  processes,  which  are  supposed  to 
eventually  occupy  the  tubes  of  the  dentin,  as  the  dentinal  fibers. 
These  cells   each    contain  an  oblong    nucleus,  which  occupies 


Pulp-cells 


Calcified  Dentin 


^^0%  'i 

Odontoblasts 


Fic.  278.— Pulp  and  Forming  Dentin  from  an  Incisor  Tooth.— (,4/?«-  Gysi.) 

the  extremity  of  the  cell  most  distant  from  the  dentin,  but  during 
the  period  of  greatest  activity  becomes  elongated  or  pointed 
in  the  direction  of  the  process.  The  odontoblastic  cells,  while 
actively  engaged  in  the  calcifying  process,  are  closely  associated 
or  crowded  together,  but  previous  to  this  time  there  is  more  or 
less  space  between  them,  which  is  filled  with  an  indifferent  tissue. 
The  first  layer  of  dentin  being  formed  upon  the  surface  of  the 
papilla,  it  will  be  observed  that  all  additions  to  its  bulk  take 
place  from  within  (the  reverse  being  true  of  enamel). 

As  stated  elsewhere,  calcification  of  the  dentin  begins  upon 
the   coronal    e.Ktremities    of   the    crowns,   the   cutting-edges    of 


420  HISTOLOGY. 

the  incisors  and  cuspids,  and  the  summits  of  the  cusps  in 
the  cuspidate  teeth  first  receiving  their  Hme-salts.  While  the 
odontoblasts  undoubtedly  superintend  the  calcifying  process,  the 
part  taken  by  these  cells  appears  to  be  somewhat  indefinitely 
determined.  It  is  generally  supposed  that  the  lime-salts  are 
secreted  under  the  superintendency  of  the  odontoblasts.  The 
secretion,  however,  does  not  take  place  around  the  cells,  and  in 


.^^ffii^^ 

^/ ,  '^^mu/P-'ij  i^^"^ 

' 

Fig.  279. — Section  through  Pulp  and  Forming  Dentin. 


that  way  completely  encapsule  them,  but  around  their  fibrils. 
While  this  is  taking  place  the  odontoblasts  remain  free  upon  the 
surface  of  the  pulp,  and  the  fibrils  assume  their  places  as  the 
organic  dentinal  fibrils.  As  the  body  of  dentin  becomes  thicker, 
the  odontoblasts  are  forced  to  recede,  and  in  so  doing  the  fibers 
lengthen.  The  dentinal  tubules  are,  of  course,  formed  in  a  like 
manner,  the  walls  of  the  tube  being  first  calcified  from  the  secre- 
tion of  lime-salts  by  the  fibrils  ;  and  as  the  fibrils  lengthen  by 


the  increasing  thickness  of  dentin  and  the  receding  of  the  odon- 
toblasts, the  tubes  also  lengthen. 

The  general  character  of  the  pulp  cells  at  a  time  immediately 
prior  to  the  appearance  of  the  ameloblasts  is  vastly  different 
from  the  same  cells  at  maturity  or  after  calcification  of  the 
dentin  has  taken  place.  The  reason  for  this  is  obvious,  con- 
sidering that  the  connective-tissue  mass  does  not  assume  its 
principal  function  until  the  odontoblasts  are  generated  about  its 
periphery.  While  at  a  later  period  the  cells  of  the  pulp  are 
oblong  in  shape,  with  slender  tail-like  processes  given  off  from 
each  end,  we  find  the  same  cells  in  the  early  embryo  (sixteenth 
to  twentieth  week)  spheroidal  in  outline  and  distributed,  as  they 
continue  to  be,  at  irregular  intervals  about  the  semi-gelatinous 
matrix.      (See  Fig.  279.) 

When  the  periphery  of  the  pulp  is  reached,  a  definite  layer 
of  cells  is  present,  corresponding  in  every  particular  to  those  of 
the  interior,  and  it  is  from  these  spheric  bodies  that  the  dentin- 
forming  cells  are  derived.  While  the  odontoblasts  are  usually 
characterized  as  spindle-  or  flask-shaped  cells,  this  can  only 
apply  to  the  cells  of  later  life,  as  those  active  at  the  beginning 
of  calcification  do  not  partake  of  either  of  these  forms.  Figure 
280  (twentieth  to  twenty-fourth  week)  shows  the  first  formed 
odontoblasts  actively  engaged  in  their  function  of  dentinification. 
It  will  be  observed  that  the  cells,  instead  of  being  individualized 
as  they  appear  at  a  later  period,  now  present  a  racemose  arrange- 
ment, such  a  cluster  appearing  about  the  entrance  to  each  den- 
tinal tubuli,  which  at  this  period  are  widely  separated  and 
apparently  without  anastomosing  branches.  The  nearer  the 
summit  of  the  crown  is  approached,  the  less  apparent  is  this 
grape-like  association  of  the  cells,  showing  conclusively  that  it 
is  a  primary  condition. 

After  a  definite  thickness  of  calcified  dentin  appears  about  the 
surface  of  the  tooth-pulp,  the  character  of  the  odontoblastic 
cells  becomes  materially  changed  (twenty-fourth  week),  but 
even  yet  they  do  not  answer  to  the  description  accorded  them. 
The  elongated,  spindle-shaped  or  club-shaped  odontoblasts  are 
without  question  found  in  connection  with  the  tissue  only  after 
calcification  has  progressed  to  a  considerable  extent ;  and  while 


422  HISTOLOGY. 

it  does  not  appear  possible  to  detect  the  minute  processes  which 
penetrate  the  calcifying  structure  before  this  stage  of  the  phe- 
nomena has  been  reached,  they  have  nevertheless  existed  from 
the  earliest  inception  of  this  specialized  layer  of  cells. 

In  this  connection  the  query  presents  itself  in  regard  to  the 
manner  in  which  the  intercommunication  between  the  dentinal 
fibers  is  established,  and  the  probable  cause  for  the  so-called  in- 


ik"v      -^»^ 

^ 

^^-T.l^. 

'   ""                -  ■  ■■  \^!iisl^M^^xKM 

^^^^^H 

'    «  .'  ^^i^l^iH 

■j 

*            '""^^vwuL'^'^jJtl^^^^^^l 

•—     ^  jj^m^w^T.yMHwB^^^^B 

^^^^^^B 

>^fyHH 

^^B-})tn^.... 

"***  ''                                  '  '  '<^'' Jfilk  ''     jA'   K^^l^^^^^^^^^^^l 

^^^^^^m 

r    'w3f\TyM^^&u^^^^^^^^^^^^K^ 

Hw 

;  I'f^^^^H 

f 

<  '"jM"  W^^^^^l^^^^r 

■  "■jIm^^^ 

Fig.  28o.-^Young  Odontoblasts  Attached  to  Forming  Dentin. 


terglobular  spaces  about  the  periphery  of  the  dentin.  The 
former  can  probably  be  explained  by  an  examination  of  the 
peripheral  cells  of  the  pulp  at  a  time  immediately  prior  to  the 
beginning  of  calcification,  when  it  will  be  found  that  these  prim- 
itive odontoblasts  communicate  with  one  another  in  a  manner 
quite  similar  to  the  canaliculi  between  the  lacunae  of  the  true 
bone,  the  connecting  processes  being  encapsuled  within  the  sub- 
stance of  the  calcifying  tissue. 


Figure  281  is  taken  from  a  very  thin  section  of  a  growing 
tooth  at  a  time  in  which  we  would  most  naturally  look  for  the 
appearance  of  the  interglobular  spaces,  and  many  such  imperfec- 
tions, if  they  may  be  so  classified,  are  observed  within  the  sub- 
stance of  the  newly  formed  tissue. 

Exceptions  might  be  taken  to  the  statement  concerning   the 


Fig,  2S1. — Section  through  Crown  of  Growing  Tooth  of  Lamb. 

racemose  appearance  of  the  early  odontoblasts  previously  re- 
ferred to,  by  claiming  the  section  to  be  one  not  directly  through 
the  long  axis  of  the  cells,  or  perhaps  transversely  through  them. 
The  examination  of  a  number  of  sections,  one  of  which  is  shown 
in  figure  282,  shows  the  cells  cut  transversely;  the  forming 
dentin  at  a  appears  with  the  tubuli  squarely  cut  off,  showing  the 


424  HISTOLOGY. 

dentinal  fibers  confined,  or  radier  appearing  as  diough  projecting 
from  the  lumen  of  the  tubes.  It  will  also  be  noted  that  the 
odontoblasts  are  irregular  in  outline,  some  of  them  being  almost 
hexagonal,  and  as  the  calcified  tissue  is  approached,  they  grad- 
ually become  reduced  in  size  and  much  modified  in  contour. 

After  a  dentin  cap  or  matrix   of  considerable  thickness  has 
made  its  appearance  and  the  enamel  cells  are  about  to  assume 


Fig.  282. — Odontoblasts  in  Transverse  Section. 


their  functional  activity,  the  odontoblasts  ior  the  first  time  begin 
to  resolve  themselves  into  the  elongated  flask-shaped  cells,  thus 
answering  the  description  usually  accorded  them,  as  illustrated 
in  figure  278.  In  fact  it  would  appear  that  they  assume  this 
shape  only  when  the  actual  lime  deposit  begins.  If  we  examine 
the  line  of  union  between  the  dentin  and  enamel  during  the 
early  growth  of  these  tissues,  it  will  be  ascertained  that,  not- 
withstanding the  dissimilarity  of  the  two  structures  at  maturity. 


there  appears  at  this  line  of  junction  a  matrix  which  may  be 
differentiated  only  by  the  free  extremities  of  the  ameloblasts,  as 
shown  in  figure  283.  When  the  enamel  matrix  begins  to  form, 
a  faint  line  of  demarcation  between  the  two  may  be  observed, 
the  difference  in  the  appearance  of  the  groundwork  of  the  two 


Fig.   283. — Section  thkough  Pulp,  Dentin,  and  Forming  Enamel.     A.   Calcified 
dentin.     B.   Pulp.      C.  Ameloblasts. 


Structures  being  one  brought  out  by  difterential  staining,  that  of 
the  enamel  taking  the  darkest  stain. 

It  will  be  observed,  therefore,  that  after  the  dentin  germ  has 
assumed  the  exact  size  of  the  dentin  of  the  future  tooth,  certain 
cells  appear  upon  its  periphery,  and  under  their  superintendence 
a  definite  laj'er  of  dentin  soon  results.     This  first  lormed  layer 


426  HISTOLOGY. 

of  dentin  is  definite  and  unchangeable  in  location,  and  it  has 
within  its  substance  the  minute  processes  from  the  dentin-form- 
ing  cells  which  are  destined  to  become  and  really  are  the  ter- 
minals of  the  dentinal  tubules.  All  who  have  given  the  subject 
of  dentin  calcification  careful  consideration  are  practically  agreed 
as  to  the  part  which  the  peripheral  pulp-cells  play  in  the  process. 
This  is  to  the  effect  that  not  about  the  body  of  the  cells  them- 
selves, but  around  their  processes  the  lime  salts  are  deposited. 
After  a  distinct  layer  of  specialized  cells  has  become  fully  estab- 
lished upon  the  very  periphery  of  the  papilla,  the  first  change 
which  takes  place  is  a  slight  withdrawal  of  these  cells  from  this 
point,  leaving  behind  slender  hair-like  processes  which  occupy  a 
portion  of  the  space  previously  taken  up  by  them,  and  about  the 
extremities  of  the  cells  and  their  processes  which  are  directed 
toward  the  enamel  organ  calcified  material  is  generated.  Zone 
upon  zone  of  calcified  dentin  appears  in  this  way,  the  body  of 
the  cell  receding,  leaving  in  its  wake  its  processes  encapsuled 
within  the  calcified  structure  as  the  dentinal  fibers. 

In  connection  with  the  primitive  layer  of  dentin-forming  cells, 
there  are  usually  described  lateral  processes  passing  from  cell 
to  cell,  apparently  serving  the  purpose  of  communication  be- 
tween the  cells.  But  these  have  recently  been  shown  to  be 
simply  a  network  of  connective  tissue  fibers  supporting  the  body 
of  the  cells.  The  theory  of  Andrews,  brought  out  some  years 
ago  in  regard  to  the  specialized  layer  of  pear-shaped  cells — 
dentin  corpuscles,  as  he  termed  them — may  be  accepted,  and 
these  should  be  considered  as  having  something  to  do  with  the 
process  of  dentinification.  The  presence  of  these  pear-shaped 
cells  at  the  beginning  of  calcification  and  during  the  continuance 
of  this  process  can  be  easily  demonstrated,  and  if  we  accept 
them  as  being  concerned  in  the  process  of  dentin  formation, 
they  might  in  a  measure  modify  the  function  now  accorded  the 
elongated  club-shaped  cells,  the  odontoblasts.  It  is  questionable 
whether  the  odontoblasts  alone  are  responsible  for  the  growth 
of  dentin  ;  they  undoubtedly  control  the  actual  process  of  lime 
deposit,  but  the  additional  cells,  no  doubt,  contribute  to  the 
structural  make-up  of  the  tissue.  It  may  be  that  by  modifying 
certain  parts  of  the  matrix,  the  result  in  the  general  structure 


CEMENTUM.  427 

is  the  dentinal  sheaths  ;  this  part  of  the  tissue  being  so  markedly 
different  from  the  bullc  of  the  intercelkilar  substance  would 
lead  us  to  believe  that  it  was  developed  from  specialized  cells. 
Further,  it  is  said  that  while  the  dentinal  tubules  are  filled  with 
a  living  substance,  this  substance  is  not  solely  the  product  of 
the  processes  of  the  odontoblasts.  That  there  is  a  special 
distribution  of  non-medullated  nerve  terminals  as  well  as  a  rich 
plexus  of  blood-vessels  about  the  periphery  of  the  pulp  is 
unquestionable,  and  this  supply  is  just  as  plentiful,  or  perhaps 
more  so,  at  maturity  as  it  is  at  the  beginning  of  calcification 
when  the  dentin  cells  are  most  active.  From  this  we  might 
be  led  to  believe  that  this  special  blood  and  nerve  supply 
to  the  periphery  of  the  pulp  is  not  solely  for  the  upbuilding  of 
the  dentin  and  therefore  distributed  to  the  peripheral  cells,  but 
also  for  the  permanent  welfare  of  the  resultant  tissue,  this  being 
brought  about  by  some  circulatory  system  throughout  the  tubules 
of  the  dentin. 

Cementum  (Fig.  284). — Investing  the  roots  of  the  teeth  is  a 
substance  which,  both  chemically  and  physically,  is  closely  allied 
to  bone.  This  external  covering  is  known  as  the  cementum,  and 
while  generally  regarded  as  being  confined  to  the  roots  of  the 
teeth,  by  some  it  is  considered  to  extend  to  and  completely 
invest  the  crowns  during  the  early  part  of  their  existence,  in 
this  latter  location  being  known  as  the  enamel  cuticle,  or  mem- 
brane of  Nasmyth. 

Generally  speaking,  the  cementum  begins  by  a  thin  margin  at 
the  neck  of  the  tooth  or  cervical  line.  It  may  commence  at  the 
free  enamel  margin  of  the  crown,  or  it  may  slightly  overlap  this 
structure.  It  is  thinnest  at  the  neck  of  the  tooth,  and  gradually 
increases  in  thickness  as  the  apex  of  the  root  is  approached. 
In  teeth  with  closely  associated  roots  the  cementum  frequently 
extends  from  one  root  to  the  other,  resulting  in  a  firm,  osseous 
union.  Histologically  considered,  the  structure  of  cementum, 
like  ordinary  bone,  consists  of  a  gelatinous,  basal  substance, 
combined  with  the  salts  of  lime,  and  of  numerous  little  hollow 
spaces — lactmcB.  Branching  in  every  direction  from  the  lacunae 
are  many  minute  processes — canaliculi. 

The  Matrix. —  The  matrix  is  so  nearly  identical  with  that  of 


428  HISTOLOGY. 

bone  that  it  is  with  difficulty  that  they  can  be  distinguished.  By 
decalcification  it  retains  its  form  and  structure,  and  by  the  inti- 
mate blending  of  organic  and  inorganic  substances  it  is  provided 
with  hardness,  solidity,  and  elasticit}\  Calcium  salts  and  col- 
lagenous fibrils,  united  by  a  small  amount  of  cement-substance, 
in  finer  or  coarser  bundles,  compose  the  ground-substance,  or 
matrix,  of  cementum. 

Let  us  first  take  up  the  study  of  this  tissue  at  different  periods 


lijgnrf^- "• '  ■  :;?^ter^ 


i 


Fig,  2S4. — Cementu.m  irom  Root  of  Molar.     X  200. 


of  its  existence,  and  in  this  manner  learn  ot  its  character,  its 
mode  of  development,  and  the  changes  which  take  place  as  its 
growth  proceeds.  The  striated  markings  of  the  tissue  have  led 
to  the  belief  that  there  are,  during  the  process  of  cementification, 
periods  of  activity  and  periods  of  rest  or  little  activity.  An 
examination  of  the  structure  under  low  power  (Fig.  285)  shows 
the  incremental  lines  placed,  with  more  or  less  regularity,  one 


CEMENTUM.  429 

beyond  the  other,   and  when    thus   studied  adds   much  to   the 
strength  of  the  theory  of  interrupted  development. 

Figure  286  is  prepared  from  a  developing  deciduous  incisor 
three  months  after  birth.  At  this  period  the  developing  organ 
is  made  up  of  enamel  and  dentin  alone,  the  process  of  cementi- 
fication  not  yet  being  under  way.  The  establishment  of  the 
dentinal  periphery,  which  surface  is  unchangeable,  provides  a 
basis  for  the  first  layer  of  cementum  generated  by  the  cemento- 
blasts,  which  at  this  period  are  forming  about  the  inner  wall  of 


Fig.  285. — Longitudinal  Section  through  Root  of  Human  Molar. 
Incremental  Likes  of  Cementum.     X  30. 


the  tooth-follicle.  In  close  proximity  to  the  surface  the  inter- 
globular spaces  are  observed  somewhat  widely  distributed,  and 
proportionately  large  in  size,  resulting  in  a  surface  poorly  calci- 
fied and  forming  a  ready  attachment  for  the  cemental  tissue. 
Figure  287  shows  the  process  of  cementification  under  way,  the 
section  being  prepared  from  a  six-month- old  tooth.  In  an  ex- 
amination of  the  ground-substance  of  this  developing  tissue  there 
is  an  unbroken  granular  appearance,  possessing  neither  stria- 
tions,  fibers,  nor  cement-corpuscles.  This  appearance  is  one 
which  persists  in  the  oldest  or  first-formed  stratum,  and  is  again 


430  HISTOLOGY. 

noticeable  in  the  outermost  or  youngest  stratum.  Wliile  the 
oldest  stratum  or  strata  retain  this  primary  character,  this  can 
not  be  said  of  those  subsequently  laid  upon  it,  for  they  succes- 
sively develop  in  their  matrix  the  partially  calcified  cells  and  fibers 
from  the  formative  tissue. 

Figure  288,  taken  from  a  one-year-old  tooth,  shows  a  further 
advance  in  the  process  of  cementification.  Many  of  the  trans- 
verse fibers  of  the  alveolodental  membrane  are  observed  pene- 
trating the  developing  tissue,  and  will,  at  a  later  period,  by  their 


-„■■  .>*^v 

Fig    286  — Section  through  Developing  Incisor,  Three  Months  after  Birth. 

X30- 

partial  calcification,  become  a  part  of  its  substance.  Already 
there  has  been  established  an  intimate  blending  of  the  cemental 
tissues  with  the  dentinal  tissues  through  the  medium  of  the 
o-ranular  layer,  and  by  the  further  calcification  of  the  latter  this 
union  gradually  becomes  more  thorough.  Figure  289  illus- 
trates three  distinct  zones  of  developing  cementum  :  the  older 
unbroken  granular  zone  at  A,  now  beautifully  cemented  to 
the  granular  layer;  a  second  or  intermediate  zone,  B,  having 
encapsuled  within  its  ground-substance  many  of  its  formative 
cells  ;  and  an   outer  zone,  C,   but  recently  laid  down,  showing 


CEMENTUM.  431 

numerous,  longitudinal,  wave-like  striations,  emblematic  of  the 
cementoblastic  activity.  In  this  outer  zone  the  minute  laminations 
disappear  as  the  tissue  becomes  more  thoroughly  calcified  and 
the  matrix  gradually  partakes  of  the  nature  of  the  older  tissue. 
The  position  occupied  by  the  cementum  on  the  root  has  much 
to  do  with  its  character.  In  the  region  of  the  cervix  the  cement- 
corpuscles  are  few  in  number,  and  when  present  possess  e.x- 
tremely  short  and  irregular  processes.  In  the  region  of  the  apex 
the  structure  is  much  more  complex  in   character,  longitudinal 


Gra.uila 
Lay« 


Terminals  r 
the  Demi 
iial  Tubuli 


Kic.  2S7. — Devkixu'Iing  Cemkntum,  from  Six-mo.nth-old  Tulith.     X  200. 


striae,  transverse  fibers,  cement-corpuscles,  and  zones  of  appar- 
ently unbroken  granular  matrix  all  serving  to  this  end. 

To  continue  the  study  of  this  tissue  let  us  exariiine  in  detail 
the  lamells,  the  cement-corpuscles,  and  the  cement-fibers. 

The  LamellcB. — We  are  told  that  the  lamellae  are  about  the 
same  in  number  over  all  parts  of  the  tooth-root,  but  that  they 
are  much  thinner  at  the  neck  than  at  the  apex.  In  addition  to 
this  they  are  usually  considered  as  running  parallel,  or  nearly 
parallel,  to  the  surface  of  the  dentin.  While  these  statements 
might,  and  probably  do,  describe  the  disposition  of  the  lamellae 


HISTOLOGY. 


in  young  cementum,  they  do  not  apply  with  so  much  certainty 
to  the  conditions  after  the  adult  period.     The  lamella;  in   the 


Fig.  2S8. — Section  through  One-ye.-vr-old  Tooth.     X  60. 


'-^^^''-.i^^^^r-^t 


^^.;-^*i 


.-^ 


Fig.  289. — Developing  Ce.me.ntum,  from  Transverse  Section  of  Bicuspid.     X  loo- 

region  of  the  apex  are  not  only  of  greater  width,  but  are  usually 
greater  in  number  than  those  occupying  the  cervix  of  the  same 
root. 


CEMEN'TUM.  433 

Figure  290  is  prepared  from  a  transverse  section  of  an  adult 
bicuspid  in  the  region  of  the  apex,  and  shows  how  the  disposi- 
tion of  the  lamella;  may  vary  in  thin,  normal  cementum.  At 
A,  which  represents  the  granular  union  of  the  cementum  with 
the  dentin,  the  incremental  lines  are  observed  to  follow  the  sur- 
face of  the  dentin.  As  the  center  of  the  area  is  approached 
this  regularity  is  much  interfered  with,  some  of  the  lamellae 
being  discontinued,  others  greatly  thickened,  while  the  field, 
taken    in    its  entirety,   exhibits  anything   but  regularity  in    the 


Fig.  290. — Transverse  Section  from  Root  of  Bicuspid,  showing  Variation  in  the 
Disposition  of  the  Lamell.s;.     )<  40- 


laying  down  of  the  different  strata.  This  same  condition  may 
be  observed  in  longitudinal  section.  While  the  lamella;  are 
usually  characteristic  of  the  cemental  tissue  in  general,  they  are 
seldom  found  in  interdentinal  cementum,  or  that  growth  which 
takes  place  between  roots,  resulting  in  their  fusion  (Fig.  291). 
This,  of  course,  refers  to  the  tissue  as  formed  between  closely 
associated  roots  of  an  individual  tooth,  and  not  to  that  union 
which  sometimes  takes  place  between  the  roots  of  different 
teeth.     The  interdentinal  tissue  previously  referred  to  appears  to 


434  HISTOLOGY. 

have  many  characteristics  common  to  itself;  thus,  the  cement- 
corpuscles  are  peculiar  in  form,  fibers  are  few  in  number,  and,  as 
before  stated,  the  lamellae  are  not  decided. 

Cement-corpuscles. — Many  of  the  cementoblasts  of  the  peri- 
dental membrane,  like  the  osteoblasts  of  the  periosteum,  become 
encapsuled  within  the  developing  tissue,  and  persist  as  irregu- 
larly shaped  spaces,  filled  with  a  protoplasmic  mass,  and  are 
known  as  cement-corpuscles.  These  correspond  to  the  lacunae 
of  bone,  but,  unlike  these,  are  verv  variable  in  size,  in  form,  and 


i^ 


Fig.  291. — Transverse  Section  through  Fused  Roots  of  Molar  Tooth, 
SHOWING  Interdentin.al  Cementum.     X  so. 


in  the  number  and  direction  of  their  processes.  Figure  284 
shows  a  number  of  cemental  lacunae  and  canaliculi.  In  the 
majority  of  instances  the  body  of  the  corpuscle  will  be  found  to 
be  oval  or  slightly  oblong,  with  its  long  a.xis  parallel  to  the  sur- 
face ;  but  it  is  by  no  means  uncommon  to  find  them  very  irreg- 
ular in  outline,  with  the  greatest  diameter  in  the  opposite  direc- 
tion. The  processes  are  quite  variable  in  length  and  irregular 
in  their  course,  and,  while  there  is  a  general  disposition  for  them 
to  extend  toward  the  surface,  they  in  many  instances  radiate  in 
various  directions.    All  of  these  features  are  in  contradistinction 


CEMENTUM.  435 

to  the  lacunae  and  canaliculi  of  bone,  which  are  placed  with  much 
more  regularity  in  the  osseous  matrix,  the  corpuscles  being  oblong 
or  cylindric  in  outline,  with  their  processes  about  equally  distrib- 
uted in  every  direction,  and  uniting  directly  and  positively  with 
the  canaliculi  of  neighboring  lacunae.  As  previously  stated,  the 
cement-corpuscles  are  very  variable  in  outline,  this  difterence  in 
form  appearing  to  be  much  influenced  by  the  part  of  the  tooth 
examined.  The  younger  corpuscles  (Fig.  292),  or  those  associ- 
ated with  the  outer  strata,  are  usually  distinctly  outlined  and 


Fig.  292. — Cement-corpuscles  of  Outer  or  Younger  Strata.     X  4°- 


provided  with  delicate  processes,  the  majority  of  which  are 
directed  toward  the  surface.  In  the  older  strata  the  outlines  of 
the  corpuscles  are  much  more  irregular,  the  processes  short  and 
extremely  clumsy. 

The  proportionate  distribution  of  the  corpuscles  to  the  various 
parts  of  the  tooth-root  is  as  follows  :  The  innermost  or  oldest 
zone  and  the  outermost  or  youngest  zones  contain  but  few  ;  in 
the  intervening  strata  they  are  most  abundant,  especially  in  the 
region  of  the  apex,  becoming  less  numerous  in  passing  crown- 
ward.     In  interdentinal  cementum  the  corpuscles  are  somewhat 


436 


HISTOLOGY. 


regularly  distributed  throughout  the  ground-substance  adjacent 
to  the  granular  layer,  but  near  the  center  of  this  confused  mass 
of  imperfectly  calcified  tissue  they  are  seldom  present.  When 
the  interdentinal  space  is  slight,  peculiarly  formed  corpuscles 
are  often  observed  (Fig.  293),  provided  with  a  long,  rod-like, 
central  portion  or  trunk,  from  which  are  given  off  numerous 
tree-like  branches,  the  terminals  of  which  are  frequently  lost  in 
the  granular  layer  upon  either  side. 

Cement-fibers. — In  a  manner    similar    to    that    in    which    the 
cementoblasts    become  encapsuled    within  the    developing    ce- 


FiG.  293. — Cement-corpuscles  Common  to  Interdentinal  Cementum.     ><  i°°- 


mental  tissue  forming  the  cement-corpuscles,  many  of  the  fibers 
of  the  peridental  membrane  undergo  a  like  transformation,  and 
are  found  in -the  tissue  as  more  or  less  imperfectly  calcified 
fibers  transversely  disposed.  By  many  writers  these  filament- 
ary, thread-like  structures  have  been  compared  to  the  delicate, 
net-like  processes  which  pass  through  the  concentric  lamellae  of 
bone,  serving  to  hold  them  together  and  designated  as  Sharpey's 
fibers ;  but,  according  to  Black,  these  are  the  principal  fibers  of 
the  alveolodental  periosteum,  and,  as  already  stated,  become  a 
part  of  the  cemental  tissue  during  its  evolution.  In  figure  294 
the    fibers    are    shown    under  high    power;    A    represents  the 


CEMENTUM.  437 

primary  or  older  stratum  of  the  tissue,  and  it  is  from  the  outer 
margin  of  this  zone  that  the  fibers  first  make  their  appearance, 
passing  more  or  less  directly  in  the  direction  of  the  surface 
until  the  next  incremental  line  is  reached,  at  which  point  they 
gradually  disappear,  but  recur  in  the  succeeding  lamellse.  There 
is  a  marked  disposition  for  the  fibers  of  each  concentric  lamella 
to  keep  within  its  borders,  or,  in  other  words,  to  become  indi- 
vidualized ;  but  in  many  instances  they  pass  .through  from  one 
lamella  to  another,  and  occasionally  extend  unbroken  through 
the  entire  thickness  of  the  tissue.     It  occasionally  happens  that 


Fig.  294. — Transverse  Section  through  Root  of  Molar,  showing  Cemental 
FiBEKS.      X  3°°- 


the  fibers  are  plentifully  distributed  to  a  region  comprising  three 
or  four  lamellje,  followed  by  a  zone  of  similar  proportions  in 
which  they  are  entirely  absent.  The  cement-fibers  considered, 
as  the  partially  calcified  residue  of  the  principal  fibers  of  the 
peridental  membrane,  would  naturally  assume  a  general  direc- 
tion relative  to  their  manner  of  distribution  before  this  change 
had  taken  place,  and  in  most  instances  they  are  thus  disposed. 
In  figure  295,  taken  from  the  center  of  a  long  axis  of  a  growing- 
bicuspid,  the  disposition  of  the  fibers,  which  are  alone  observed 
in  the  second  lamella,   is  slightly  crownward.     The  inclination 


438 


HISTOLOGY. 


for  the  fibers  to  be  thus  disposed  is  most  pronounced  in  young 
cementum,  but  after  middle  life,  or  at  a  period  when  the  tissue 
has  greatly  increased  in  thickness,  the  course  of  the  fibers,  even 
in  the  same  locality,  is  greatly  at  variance. 

In  figure  28S,  also  from  a  young  tooth,  the  fibers  are  shown 
springing  directly  from  the  peridental  membrane,  with  their 
free  extremities  penetrating  this  tissue.  This  illustration  is  pre- 
pared from  a  transverse  section  in  the  cervical  region,  and  the 
inclination  of  the  fibers  is  such  as  to  warrant  the  belief  that 
they  were  some  ot  those  whose  function  it  has  been  to  return 


■    '  «  '"*¥ s^  .a""- 


the  tooth  to  its  normal  position  when  slightly  rotated  upon  itself. 
Another  class  of  fibers  common  to  the  cement-tissue  are  those 
which  appear  to  be  grouped  in  bundles,  springing  more  or  less 
regularly,  at  intervals,  from  the  granular  layer  and  penetrating 
the  basement  layer  of  the  cementum  as  though  serving  to  tie 
this  tissue  to  the  periphery  of  the  dentin.  In  figure  296  a 
number  of  these  bundles  are  shown  at  A,  B,  and  C.  While 
the  field  is  but  a  small  proportion  of  the  circumference  of  the 
root,  they  are  observed,  under  low  power,  to  be  distributed  in  a 
like  manner  to  all  parts.     These  circumferential  fibers,  as  they 


CEMENTUM.  439 

may  be  called,  are  also  observed  in  longitudinal  section,  being 
distributed  with  considerable  regularity  throughout  the  whole 
extent  of  the  root.  They  are  also  present  in  the  tissue  at  the 
earliest  period  at  which  its  character  may  be  studied,  the  indi- 
vidual bundles  at  this  time  being  proportionately  larger.  These 
might  be,  and  probably  are,  considered  as  prolongations  from 
the  dentinal  fibers,  but  it  is  doubtful  if  the  true  fibers  of  the 
dentin  are  ever  found  penetrating  the  cementum. 

Cementification. — We  have  seen  in  the  study  of  the  develop- 


ment of  the  teeth,  that  the  tooth-generating  organs  were  confined 
in  a  closed  sac  or  follicle,  and  while  the  walls  of  this  sac  were 
not  directly  interested  in  the  calcification  of  the  dentin  or  enamel, 
this  can  not  be  said  of  the  cementum.  Attention  has  also  been 
directed  to  the  fact  that  at  the  time  of  the  eruption  of  the  crown 
of  the  tooth  a  portion  of  the  root  only  is  calcified.  As  the  growth 
of  the  root  continues,  the  follicular  wall  becomes  closely  adherent 
to  it.  Upon  the  inner  face  of  this  vascular  membrane  a  layer  of 
osteoblastic  cells  (cementoblasts)  is  generated,  and  as  a  result 
of  the  calcification  of  these  cells  the  cementum  is  formed.      It 


440  HISTOLOGY. 

will  thus  be  seen  that  the  process  of  cementification  is  but  a 
slightly  modified  form  of  subperiosteal  bone  development.  At 
the  beginning  of  cementum  calcification  the  diameter  of  the 
dentin  of  the  root  is  as  great  as  it  will  ever  be,  all  additions  to 
its  bulk  taking  place  from  within.  But  while  the  diameter  of  the 
dentin  is  thus  fixed,  the  diameter  of  the  root  is  increased  by  the 
additional  layers  of  cementum  as  they  are  deposited  upon  its 
surface.  As  previously  stated,  a  single  layer  of  cementoblasts  is 
first  formed  in  the  membrane  surrounding  the  root,  these  soon 
becoming  inclosed  in  a  spherule  of  lime.  By  the  time  this 
has  taken  place  another  layer  makes  its  appearance,  assuming 
all  the  characteristics  of  the  first  formed  layer.  Other  layers 
are  formed  in  turn  until  the  cementum  assumes  its  mature 
thickness. 

The  Dental  Pulp  (Fig.  297). — The  tooth-pulp,  or  formative 
organ  of  the  dentin,  occupies  the  central  or  pulp-cavity,  and  in 
the  fully  developed  tooth  assumes  a  general  outline  closely  cor- 
responding to  the  exterior  of  the  organ.*  Along  with  its  pri- 
mary function  of  generating  the  dentin,  it  becomes  the  medium 
through  which  this  structure  receives  its  vascular  and  nervous 
supply. 

Histologically  considered,  the  pulp  may  be  described  as  a 
mucus-like,  protoplasmic  matrix,  containing  delicate  connective- 
tissue  fibers  not  formed  into  bundles  and  numerous  nucleated 
cells,  the  latter  being  especially  numerous  on  the  peripherj^  of 
the  pulp,  or  that  portion  which  comes  in  contact  with  the  dentin. 
The  cells  are  not  closely  enough  associated  to  form  a  complete 
tissue  in  themselves,  but  are  found  embedded  in  a  mucoid  matrix, 
with  always  a  definite  space  between  them.  In  general  the  cells 
are  elongated  or  spindle-shaped,  with  a  delicate,  hair-like  process 
attached  to  either  extremity.  In  the  pulp-chamber  the  cells  vary 
somewhat  in  outline,  in  some  instances  being  spheroid,  in 
others  appearing  as  slender  filaments,  so  that  the  cell  proper 
can  scarcely  be  distinguished  from  its  processes.  A  third 
class  of  cells  may  be  met  with,  from  which  three  or  more  fila- 

*  The  pulp  not  only  occupies  the  central  cavity  in  the  tooth-crown,  but  the  canals  of  the 
roots  as  well;  therefore  the  form  of  the  pulp  corresponds  to  the  outline  of  the  pulp-cavity, 
already  described. 


THE  DENTAL  PULP.  441 

merits  are  given  off.  As  stated,  the  distribution  of  cells  varies 
considerably  in  different  parts  of  the  pulp,  this  being  true  not 
only  as  regards  numbers,  but  also  as  to  the  relations  existing 
between  the  cells.  In  the  coronal  portion  of  the  pulp  the  posi- 
tion assumed  by  each  individual  cell  appears  to  be  without 
regard  to  the  position  of  neighboring  cells,  while  in  that  portion 
of  the  pulp  occupying  the  root-canals  the  cells  are  arranged 
parallel  with  the  length  of  the  root.     The   cells  are   least  in 


Fig.  297. — Longitudinal  Section  through   Human  Cuspid,  showing  Tooth-pulp.- 
{Afler  Gvsi.)     X  10 


number  in  the  interior  of  the  pulp,  but  gradually  become  more 
plentiful  as  the  periphery  is  approached.  (See  Cells  of  Dentin 
Papilla,  page  305.) 

The  Odontoblasts  (Fig.  298). — The  most  active  cells  of  the 
pulp  are  those  direcdy  on  its  periphery,  in  contact  with  the 
dentin,  and  known  as  the  odontoblasts.  The  odontoblastic 
layer,  otherwise  known  as  the  membrana  eboris,  is  composed  of 
a  single  row  of  cells,  each  of  which  contains,  near  the  extremity 
most  distant  from  the  dentin,  a  well-defined  nucleus.     They  are 


442  HISTOLOGY. 

large,  elongated  cells,  each  furnished  with  three  sets  of  fibers 
or  processes — the  dentinal  process,  the  pulpal  process,  and  the 
lateral  process.  The  dentinal  process  or  processes — there 
may  be  more  than  one  present — communicate  with  the  deeper- 
lying  cells  of  the  pulp,  while  by  means  of  the  lateral  processes 
the  cells  are  brought  Into  communication  with  neighboring  cells. 
The  processes  given  off  In  the  direction  of  the  dentin,  or  the 


Fig.  298. — Transverse  Section  through  Pulp.     Blood-vessels  and  Nerves 
Cross-cut. 


dentinal  processes,  may  be  one  for  each  cell.  In  which  case  they 
are  of  considerable  size,  and  are  inclined  to  taper  as  they  enter 
the  substance  of  the  dentin.  Again,  a  single  cell  may  give  off 
a  number  of  smaller  processes  in  this  direction.  The  odonto- 
blasts vary  much  in  form  according  to  their  functional  activity. 
Before  the  period  of  dentlnification  they  are  spheroid'  or  pyri- 
form,  during  the  period  of  calcification  the  dentin  extremity  be- 
comes somewhat  flattened  and  square,  while  in  advanced  years 


THE   DENTAL   PULP.  443 

they  again  return  to  their  primitive,  rounded  form.  Covering 
the  entire  surface  of  the  pulp  Hlce  an  epithehum,  the  odonto- 
blasts are  especially  closely  associated  at  the  end  nearest  the 
dentin,  forming  an  unbroken  layer,  while  the  pulpal  extremities 
are  inclined  to  assert  their  individuality  by  disassociation. 

Blood-vessels  of  the  Pulp. — The  pulp   is  richly  supplied  with 
blood-vessels,    forming    networks    extending    principally    in    a 


Fig.  299. — PuLr  and  Dentin  in  Longiiudinal  Section. 


direction  parallel  to  the  long  axis  of  the  tooth,  and  finally 
terminate  in  a  capillary  plexus  closely  associated  with  the  odon- 
toblastic layer.  The  veins  of  the  pulp  are  ordinarily  somewhat 
larger  than  the  arteries,  and  form  numerous  anastomoses.  This 
organ  appears  to  be  destitute  of  lymphatics — at  least,  none  are 
known  to  occur  in  Its  substance.  The  blood-vessels  of  the 
pulp  are  provided  with  a  longitudinal  layer  of  thinly  distributed 


HISTOLOGY. 


muscular  fiber,  but  otherwise  the  walls  of  the  vessels  are  noted 
for  their  delicacy. 


Ftg.  30c. — Section  through  Pulp.     Fig.  299  in  Transverse  Section. 


Nei^ves  of  the  Pidp. — After  entering  the  apical  foramen  either 
by  one  large  trunk  or  by  two  or  more  minute  ones,  the  fibers 
pursue  a  parallel  course,  breaking  up  but  little  or  giving  off  but 
few  fibers  in  that  portion  of  the  pulp  confined  to  the  canal. 
When  the  expanded  or  coronal  portion  of  the  pulp  is  reached, 
numerous  subdivisions  occur  which  are  distributed  in  every 
direction,  and  ending  in  a  rich  plexus  beneath  the  odontoblastic 
layer,  or  membrana  eboris.  In  the  body  of  the  pulp  the  fibers 
are  medullated,  but  those  occupying  the  periphery  are  non- 
medullated  and  supposed  to  pass  into  the  dentinal  tubes.  While 
this  latter  hypothesis  is  in  all  probability  correct,  such  a  distri- 
bution of  the  germinal  fibers  has  never  been  definitely  demon- 
strated.    Two  investigators  (Ball  and  Maoitot)  claim  to  have 


NASMYTH'S   MEMBRANE,  445 

partially  satisfied  themselves  in  regard  to  the  final  distribution 
of  the  non-medullated  fibers.  The  former  states  that  he  has 
traced  these  fibers  into  continuity  with  the  larger  medullated 
fibers  in  the  deeper  pulp-tissue,  and  claims  to  have  found  them 
passing  through  the  membrana  eboris,  beyond  which  point  they 
assumed  a  direction  parallel  to  the  dentinal  tubules.  This  theory 
is  controverted  by  Magitot,  who  claims  that  the  dentinal  fibers 
are,  in  a  measure,  themselves  prolongations  of  the  nerves,  being 
so  constituted  through  the  medium  of  the  branched  stellate  cells 


Branching  ol  Main 
Nerve-trunk  into 
Single  Fibers 


hing  of  Main 
Blood-vessels  into 
Capillaries 


Fig.  301. — DisTRiBUTinN  of  Blood-vessei.s  and  Nerves  to  the  Pulp  of 
Human  Molar.  —  [AfU'i-  Gysi.)     X  20. 


which  lie  immediately  beneath  the  membrana  eboris,  and    by 
which  the  nerves  are  made  continuous. 

Nasmyth's  Membrane. — Nasmyth's  membrane,  otherwise 
known  as  the  enamel  cuticle  or  persistent  dentinal  capsule,  is 
an  exceedingly  thin  and  peculiarly  indestructible  structure,  en- 
tirely covering  the  enamel.  As  to  the  presence  of  this  mem- 
brane, which  can  be  demonstrated  only  by  chemic  detach- 
ment, there  appears  no  doubt,  but  in  regard  to  its  origin  and 
definite  structure  much  difference  of  opinion  has  been  ex- 
pressed.     By  some  writers  (Tomes  and   Magitot)   it  is  main- 


446  HISTOLOGY. 

tained  that  it  is  continuous  with,  and  similar  in  structure  to,  the 
cementum  covering  the  root,  being  an  extension  of  the  outer- 
most layer  in  the  region  of  the  neck  of  the  tooth ;  and,  in  view 
of  the  fact  that  lacunae  are  found  in  its  substance,  this  theory 
would  appear  to  be  correct.  On  the  other  hand,  it  is  considered 
to  be  a  product  of  the  epithelium  (Huxley  and  Kolliker)  and  in 
no  manner  connected  with  the  cementum.  In  the  opinion  of 
the  author,  it  would  be  difficult  to  understand  how  the  theory 
advanced  by  Tomes  could  be  accepted.  During  the  entire 
period  of  saccular  development  the  crown  of  the  tooth  is  in 
close  relationship  to  the  enamel  organ,  this  structure  intervening 
between  the  forming  enamel  and  the  wall  of  the  tooth-sac,  from 
which  the  cementum  is  developed.  It  would,  therefore,  appear 
that  this  membrane  is  generated  from  the  external  epithelial 
layer  of  the  enamel  organ  by  a  change  in  the  character  and  form 
of  these  cells.  Sudduth  attributes  its  tormation  to  a  metamor- 
phosis of  the  ameloblastic  layer,  the  prismatic  cells  assuming  a 
horizontal  direction.  The  ameloblasts  are  observed  to  be  pris- 
matic in  form  up  to  the  point  at  which  the  enamel  prisms  are  yet 
unfinished,  but  as  the  surface  is  approached  they  are  observed  to 
shorten  and  widen,  and  near  the  gum-margin  they  assume  a  longi- 
tudinal direction  instead  of  being  at  right  angles  to  the  body  of 
the  crown.  Mrs.  Emily  Whitman  has  devoted  much  time  to  the 
study  of  the  development  of  mammalian  teeth,  and  appears  to  be 
of  the  opinion  that  the  cuticula  dentis  is  the  result  of  a  change 
in  the  form  and  character  of  the  enamel  cells,  this  metamor- 
phosis taking  place  either  before  or  after  calcification  of  the 
underlying  tooth-tissues.  Nasmyth's  membrane  shows  many 
characteristics  which  differ  from  those  of  the  body  of  enamel 
subjacent  to  it,  serving  as  an  indestructible,  highly  polished 
surface-capping  to  the  enamel  prisms.  The  indestructible  nature 
of  this  membrane  by  reagents  would  appear  to  indicate  that  in 
structure  it  is  closely  akin  to  the  structure  lining  the  dentinal 
tubules,  the  lacunse,  etc. 

Alveolodental  Membrane  (Fig.  300).  —  As  a  general 
description  of  this  membrane  has  already  been  given  in  part 
I,  it  alone  remains  to  treat  of  its  histologic  character,  which 
may    best    be    accomplished    by   first    referring    to  the    duties 


ALVEOLODENTAL   MEMBRANE.  447 

which  it  has  to  perform.  These  may  be  divided  into  three 
classes — functional,  physical,  and  sensory.  The  functional  office 
is  accomplished  through  its  cellular  elements — the  osteoblasts 
and  cenientoblasts ;  the  physical  office  is  performed  by  the 
fibrous  elements,  through  which  the  tooth  is  fixed  in  its  position  ; 
and  the  sensory  office  through  the  abundance  of  nerves,  which 
are  richly  distributed  to  all  parts  of  the  membrane.  We,  there- 
fore, find  in  this  structure,  besides  connective  tissue,  cells,  fibers, 
nerves,  and  blood-vessels.     The  principal  cells, as  already  stated. 


'^H^i 


Alveolar  Wall 


Fig.  302, — Transverse   Section  through    Root  of  Human   Incisor   and   Surround- 
.     ING  Alveolar  Wall,  with  Alveolouental  Membrane  Intervening.     X  4°- 


are  the  osteoblasts  and  cementoblasts,  but  there  are  also  present 
fibroblasts  and  osteoclasts. 

TJie  osteoblasts,  which  are  instrumental  in  the  upbuilding  of  a 
portion  of  the  alveolar  walls,  are  found  lying  against  the  bone, 
between  the  principal  fibers.  These  cells  do  not  appear  to  be 
evenly  distributed,  being  numerous  and  crowded  together  in 
some  parts,  while  others  will  appear  to  be  almost  destitute  of 
them.  They  are  most  plentiful  in  the  young  subject,  and  seldom 
present  at  all  in  old  age.  In  youth  the  alveolodental  membrane 
is  thickest,  and,  as  the  building  of  bone  occurs  on  the  inner  wall 


HISTOLOGY. 


of  the  alveolus,  it  can  only  progress  as  the  membrane  becomes 
reduced  in  thickness.  The  osteoblasts  are  polygonal  cells,  in- 
clining to  the  oval  form,  and  vary  greatly  in  size,  with  their 
longest  diameter  at  right  angles  with  the  surface  of  the  forming 
bone.  During  the  period  of  the  development  of  the  young 
alveolar  wall  they  are  inclined  to  be  crowded  together,  and  are 
frequently  much  distorted  from  pressure  upon  one  another.  As 
age  advances  this  condition  becomes  less  pronounced,  and  the 
cells  separate  into  groups. 


Fig.  303. — Section  through  Root  of   Tooih,  Alveolodental  Memhkane,  and 
Alveolus. 


The  Cenientoblasts . — Stationed  upon  the  opposite  side  of  the 
membrane,  or  that  in  contact  with  the  root  of  the  tooth,  are 
another  class  of  cells — the  cementoblasts — or  those  cells  which 
are  concerned  in  the  formation  of  the  cementum.  Like  the 
osteoblasts,  these  cells  are  found  lying  between  the  principal 
fibers  of  the  root-membrane.  They  differ  in  form  from  the 
osteoblasts,  notwithstanding  that  they  have  a  similar  function. 
Instead  of  the  polygonal  form   common   to  the  osteoblasts,  we 


ALVEOLODENTAL   MEMBRANE.  449 

find  these  cells  to  be  more  or  less  flattened,  with  outlines  some- 
what irregular.  Extending  from  the  body  of  the  cell,  which 
contains  a  well-defined  nucleus,  are  a  number  of  irregular  pro- 
cesses, which  penetrate  the  neighboring  fibers  or  the  interfibrous 
substance.  Unlike  the  osteoblasts,  the  cementoblasts  appear 
at  all  times  to  be  evenly  distributed  over  the  surface  of  the 
cementum,  occupying  all  the  space  except  that  taken  up  by  the 
fibers  as  they  leave  the  cementum.     As  to  the  development  of 


Epitheliu 


Alveolo- 
Hental 
Membrane 


Fig.  304. — Transverse  Section  through  Root  of  Tooth,  Alveolodental  Mem- 
brane, Thin  Wall  of  Alveolus,  and  Gingival  Tissue. 


the  osteoblasts  and  cementoblasts,  they  appear  to  be  carried  to 
the  fibrous  meshes  of  the  membrane  by  the  blood  as  leukocytes 
or  ameboid  cells,  after  which,  by  differentiation,  they  become 
fitted  for  the  development  of  bone  or  cementum,  assuming  their 
respective  places  against  the  surface  of  one  or  the  other  of 
these  structures. 

Fibroblasts  and  Osteoclasts. — Fibroblasts  and  osteoclasts  are 
also  present  in  the  alveolodental  membrane,  the  former  for  the 
purpose  of  the  increase  or  renewal  of  the  fibrous  tissue,   the 
29 


450  HISTOLOGY, 

latter  being  functionally  concerned  in  the  removal  of  a  part  of 
the  alveolar  walls  to  accommodate  the  ever-varying  position 
of  the  teeth,  or  acting  in  a  similar  manner  upon  the  cementum  of 
the  root.  The  osteoclasts,  or  giant-cells,  are  generally  inclined 
to  the  round  or  oblong  form,  and  usually  contain  a  number  of 
nucleoli.  They  vary  much  in  size,  and  are  seldom  branched  or 
provided  with  processes.  In  addition  to  the  four  classes  of 
cells  already  mentioned  as  being  present  within  the  meshes 
of  the  fibrous  tissue  of  the  root-membrane,   there  is  another 


Fig.  305. — Longitudinal  Section  through  Root  of  Growing  Tooth  near  the 
Cervix. 


class,  present,  however,  during  youth  only,  which  appears  to 
be  in  course  of  development,  and,  therefore,  without  apparent 
function. 

The  Fibe7^s  of  the  Alveolodental  Membrane. — The  principal  fi- 
bers of  the  alveolodental  membrane  are  those  which  extend  from 
the  cementum  on  one  side  to  the  alveolar  wall  on  the  other,  and 
become  firmly  fixed  at  either  extremity  by  penetrating  the  cal- 
cified structures.  The  fibers  are  all  of  the  white,  or  inelastic, 
connective-tissue  variety.  It  Is  by  means  of  the  connective-tissue 
fibers  that  the  actual  attachment  of  the  membrane  both  to  the 


ALVEOLODENTAL    MEMBRANE.  451 

bone  and  to  the  cementuni  takes  place,  the  fibers  passing  directly 
into  the  hard  tissues,  which  they  traverse  for  some  distance, 
being  here  known  as  Sharpey's  fibers. 

The  arrangement  of  the  fibers  is  somewhat  dififerent  over 
the  various  parts  of  the  root.  In  the  region  of  the  gingival 
margin  they  pass  out  from  the  substance  of  the  cementum, 
retaining  their  solid  form  or  dividing  into  fasciculi  of  finer 
fibers.  In  general  the  fibers  lie  parallel  with  one  another,  devi- 
ating only  to  give  place  to  blood-vessels  and  nerves.     There 


-LO.^GITUDINAL   Sf.CTION   THROUGH   ALVEOLODENTAL   MEMIRANE,    GINGIVAL 

Tissue,  and  Root  of  Tooth.     Cervical  District. 


is  some  variation  in  the  distribution  of  the  fibers  about  the 
different  gingival  surfaces.  Upon  the  labial  and  lingual 
surfaces  they  pass  out  directly  into  the  fibrous  tissue  of  the 
gum,  and  soon  become  lost  in  this  tissue.  On  the  mesial 
and  distal  surfaces  the  fibers  passing  the  lower  margin  of  the 
alveolar  wall  join  the  fibers  of  the  neighboring  tooth.  This 
disposition  for  the  fibers  to  bend  toward  the  adjacent  tooth  is 
first  observed  at  the  various  angles  of  the  gingival  margin.  All 
about  the  free  border  of  the  eum  the  fibers  from  the  alveolodental 


452  ,.  HISTOLOGY. 

membrane  assist  in  forming  this  tissue,  which  is  covered  by  a 
dense  epithelial  coating  of  moderate  thickness,  surrounded  or 
surmounted  by  the  peridental  fibers.  As  the  border  of  the  al- 
veolar wall  is  approached,  the  fibers  are  observed  to  pass  under 
the  proper  tissues  of  the  gum,  and  unite  with  the  outer  perios- 
teal layer  overlying  the  outer  alveolar  wall.  The  fibers  imme- 
diately within   the  alveolus  are   slightly  inclined   in  an   apical 


Fig.  307. — Section  showing  Fibers  of  Alveolodental  Membrane,  Attached  to 
AND  Passing  Out  from  the  Cementum. 


direction,  while  those  occupying  the  central  portion  of  the 
membrane,  or  that  midway  between  the  apex  and  the  gingiva, 
pass  nearly  straight  across  from  the  cementum  to  the  bone.  It 
is  in  this  locality  that  the  largest  and  strongest  fibers  are  found. 
As  the  apex  of  the  root  is  approached  the  inclination  of  the  fibers 
is  crownward  from  the  cementum  to  the  alveolar  wall.  In  this 
situation  the  single  fibers  are  inclined  to  break  up  the  fasciculi. 


ALVEOLODENTAL   MEMBRANE. 


453 


Immediately  surrounding  the  apex  of  the  root  the  fibers  are 
irregular  during  youth,  but  are  disposed  more  regularly  or  fan- 
like in  older  subjects. 

While  this  account  briefly  furnishes  a  description  of  the  distri- 
bution of  the  fibers  in  various  locations,  and  is  in  most  instances 
correct,  they  occasionally  vary  from  this  arrangement.  While 
in  most  respects  the  fibers  of  this  membrane  closely  resemble 
the  corresponding  fibers  of  attached  periosteum,  they  possess 
some  peculiarities.     It  might  be  supposed  that  the  fibers  passing 


Fig.  30S. — Transverse  Section  through  Growing  Roots  and  Alveolus. 


out  from  the  cementum  would  in  some  way  difter  from  those 
springing  from  the  alveolar  wall,  but,  with  the  exception  ot 
being  somewhat  less  in  size,  they  ai'e  otherwise  of  the  same 
character. 

Interfibrous  Elenieiits. — Besides  the  various  forms  of  cells, 
blood-vessels,  and  nerves,  there  is  present  in  the  alveolodental 
membrane  an  interfibrous  tissue.  This  tissue  is  principally 
composed  of  the  fibroblasts  belonging  to  the  principal  fibers, 
and  other  fibroblasts  accompanied  by  delicate  fibers  which 
appear    to    be     independently    distributed.       This    interfibrous 


454  HISTOLOGY. 

tissue,  which  is  thus  seen  to  be  ordinary  fibrous  connective 
tissue,  appears  to  pervade  the  entire  membrane  wherever  suf- 
ficient space  is  found  to  permit  of  its  presence.  In  some  parts 
of  the  membrane  this  tissue  appears  to  be  more  plentiful  than 
the  principal  fibers  themselves.  The  interfibrous  tissue  also 
forms  an  investment  for  the  blood-vessels  and  nerves  in  addition 
to  the  tissues  properly  belonging  to  their  walls. 


CHAPTER  VI. 
EMBRYOLOGY  OF  THE  MOUTH  AND  TEETH. 

In  the  consideration  of  this  subject  it  will  be  appropriate  to 
briefly  refer  to  general  embryology,  which  includes  an  investi- 
gation into  the  mode  of  formation  and  the  gradual  development 
•  of  the  animal  fetus.  The  history  of  the  subject  is  replete  with 
well-defined  epochs,  each  of  which  represents  for  a  period  the 
theories  or  discoveries  of  some  well-known  investigator.  While 
not  a  few  of  the  older  histologists  were  well  informed  in  regard 
to  very  many  important  embryologic  facts,  their  opinions  varied 
greatly,  and  they  were  all  for  a  time  inclined  to  misinterpret 
certain  vital  points,  and  it  was  principally  this  latter  fact  which 
stimulated  and  encouraged  further  research. 

Aristotle  held  views  which  were  for  a  long  time  accepted  as 
correct,  his  hypothesis  being  that  all  living  creatures  were  gene- 
rated in  one  of  three  ways :  /.  e.,  the  egg-  was  deposited,  incu- 
bated, and  hatched  (oviparous),  or  a  fully  formed  and  independent 
offspring  was  brought  forth  alive  (viviparous),  or  a  third,  in 
which  the  being  was  accorded  a  spontaneous  birth,  this  latter 
class  including  the  lower  order  of  animals,  parasites,  insects, 
etc.  Little  or  no  exception  was  taken  to  the  first  and  second 
modes  mentioned,  but  by  persistent  investigation  it  was  gradu- 
ally determined  that  the  number  of  occurrences  in  which  genera- 
tion took  place  spontaneously  was  much  less  frequent  than  had  at 
first  been  supposed.  In  fact  it  was  for  a  time  acknowledged 
that  spontaneous  generation  was  never  responsible  for  the 
development  of  insects  or  animalcules.  More  recently  the  sub- 
ject has  again  been  taken  up,  and  it  is  conceded  at  the  present 
time  that  certain  forms  of  uncertain  infusoria  are  reproduced 
by  a  spontaneous  subdivision  of  unicellular  organisms  (fission, 
fissipara). 

Up  to  the  time  of  Harvey  (1631)  there  existed  a  misunder- 
455 


456  EMBRYOLOGY. 

Standing  in  regard  to  the  similarity  existing  in  the  generation  of 
species,  be  they  of  oviparous  or  viviparous  origin.  Previous  to 
this  there  was  supposed  to  be  a  distinct  difference  in  the  two 
modes  of  development,  but  Harvey's  bold  assertion  that  "all 
animals  whatsoever,  even  the  viviparous  and  man  himself  not 
excepted,  are  generated  from  ova,"  revolutionized  the  study  of 
embryology,  and  established  a  fundamental  principle  (the  mag- 
nitude and  value  of  which  was  never  known  to  him)  through 
which  all  subsequent  investigations  were  made  blessed. 

It  is  found,  therefore,  that  the  origin  and  a  considerable  por- 
tion of  the  early  development  of  the  embryo  take  place  in  a 
similar  manner  in  all  animals,  from  the  impregnated  egg  of  the  ' 
female,  and  this  being  the  case  we  are  particularly  favored  in 
our  early  investigations  by  using  the  egg  of  a  fowl.  Therefore 
attention  will  be  briefly  directed  to  the  history  of  the  develop- 
ment of  the  chick. 

Upon  examining  a  longitudinal  section  of  a  hen's  egg,  in  pass- 
ing from  the  periphery  toward  the  center  the  following  struc- 
tures are  found  :  First,  that  part  commonly  known  as  the  shell, 
which  is  made  up  of  variable  proportions  of  organic  and  inor- 
ganic substances.  To  provide  for  the  early  process  of  respira- 
tion this  structure  is  amply  porous  to  permit  the  free  passage  of 
gases  from  without  inward,  and  vice  versa.  The  shell  is  made 
up  of  two  distinct  layers,  but  these,  of  course,  are  not  directly 
or  indirectly  interested  in  the  development  of  the  embryo.  Im- 
mediately within  the  shell  is  the  shell  membrane,  wdiich  is  like- 
wise composed  of  two  layers.  These  two  layers  are  closely 
associated  throughout  their  entire  extent  except  in  the  district 
represented  by  the  broad  end  of  the  egg ;  here  they  exhibit  a 
marked  tendency  to  separate,  the  outer  layer  remaining  in  close 
contact  with  the  inner  wall  of  the  shell,  while  the  inner  layer  re- 
mains as  a  direct  covering  to  the  white  of  the  egg  proper.  In 
this  manner  an  air  chamber  is  formed  which  usually  increases 
with  the  age  of  the  egg. 

Immediately  within  this  is  what  is  commonly  known  as  the 
white  of  the  egg,  which  upon  careful  examination  exhibits  a  sort 
of  laminated  appearance. 

The  next  layer  is  known  as  the  yolk,  or  that  part  which  is 


THE   BUCCAL   CAVITY.  457 

essential  to  the  production  of  the  embryo,  inclosed  by  a  thin  layer 
known  as  the  vitelline  membrane. 

To  simplify  our  study,  let  us  banish  from  mind  all  other  parts 
of  the  egg  and  continue  by  describing  the  early  changes  which 
take  place  in  the  yolk.  In  fact,  it  will  be  well  at  this  time  to 
consider  that  we  have  under  discussion  the  development  of  an 
embryo,  not  particularly  of  the  fowl,  but  of  the  mammalia  as 
well.  This  spherical  body  may  be  compared  to  a  single  cell, 
that  part  which  is  known  as  the  vitelline  membrane,  or  zona 
pellucida,  corresponding  to  the  cell  wall ;  the  body  of  the  yolk,  or 
vitellus,  representing  the  cell  protoplasm,  within  this  the  germi- 
nal vesicle,  for  the  cell  nucleus,  while  the  individualized  central 
area  of  the  vesicle  is  given  up  to  the  nucleolus  or  germinal 
spot.  Without  further  considering  this  primitive  cell,  let  us  pass 
on  to  a  description  of  the  germinal  membrane  known  as  the 
blastodemn.  The  time  at  which  this  membrane  makes  its  ap- 
pearance is  so  variable  in  the  different  species  that  no  allusion 
will  be  made  to  this  part  of  the  subject.  Primarily  the  blasto- 
derm which  lies  upon  the  internal  surface  of  the  vitelline  mem- 
brane is  composed  of  two  layers,  and  from  these  two  layers  the 
embryo  is  generated.  In  a  very  short  time  a  third  layer  makes 
its  appearance,  not,  as  might  be  supposed,  upon  one  side  or  the 
other  of  those  already  present,  but  between  them,  constituting 
the  three  primary  layers  of  the  germinal  membrane — the  epi- 
blast,  mesoblasi,  and  hypoblast.  These  three  layers  are  found 
in  connection  with  the  early  embryo  of  all  vertebrates,  if  not  in 
all  invertebrates. 

Following  carefully  the  development  of  the  embryo  from  this 
time,  we  find  that  the  outer  layer  of  cells  (epiblast)  is  productive 
of  the  following  tissues  of  the  mouth  :  the  epithelial  lining  of  the 
cavity,  and  the  enamel.  From  the  middle  layer  (mesoblast) 
arises  the  dentin,  all  the  connective  tissue,  blood-vessels,  and 
lymphatics,  together  with  the  bones  which  form  the  framework 
of  the  mouth,  while  the  inner  layer  (hypoblast)  gives  origin  to 
the  epithelial  lining  of  the  alimentary  canal  other  than  the  oral 
cavity. 

The  Buccal  Cavity. — The  early  appearance  of  the  entrance 
to  the  alimentary  canal  is  found  in   the  formation  of  an  open 


458  EMBRYOLOGY. 

cavity  bounded  by  the  primitive  maxillary  processes  above  and 
mandibular  arch  below. 

The  cavity  thus  formed  is  the  common  buccal  space,  the 
upper  portion  being  the  respiratory  or  nasal  section,  while 
below  is  the  true  mouth.  The  cavity  of  the  mouth,  as  such, 
does  not  exist  until  these  two  are  completely  separated  by  the 
palatal  plates  forming  the  future  roof  of  the  mouth. 

Figure  309  shows  a  vertical  transverse  section  through  this 
common  buccal  cavity.     At  this  early  period  the  lateral  walls 


/i    Dental  Ridg 
//       Lowerjaw 


Fig.  309. — Vertical  Transverse  Section  through  Head  of  Human  Embryo, 
ABOUT  the  Tenth  Week.     X  3°- 


and  floor  ot  the  mouth  are  manifest  by  certain  cellular  elements, 
but  the  roof  of  the  cavity,  as  already  stated,  is  not  complete 
until  the  palatal  plates,  now  separated  by  the  tongue,  grow 
inward  and  unite  at  the  median  line. 

The  Oral  Cavity.— The  Roof  of  the  Mouth.— When 
these  two  processes  which  arise  from  the  mesoblast  unite  at  the 
median  line,  they  establish  a  permanent  horizontal  septum, 
dividing  this  part  of  the  stomodeum  into  a  respiratory  or  nasal 
section  and  an  oral  section,  the  mouth.     The  cells  entering  into 


THE    ORAL   CAVITY. 


this  part  of  the  fetal  head  at  this  time  (eighth  to  tenth  week) 
are  of  three  varieties,  being  connective-tissue  cells,  cartilage 
cells,  and  epithelial  cells,  the  latter  being  distributed  in  a  layer 
of  varying  thickness  over  those  parts  destined  to  become  a 
part  of  the  lining  membrane  of  the  mouth. 

In  figure  310  (twelfth  week)  the  superior  maxillary  processes 
are  shown  united  and  the  permanent  separation  between  the 
mouth  and  nasal  cavity  established.    This  embryonal  bridge  is  for 


Meckel's  Cartilag 


Denial  Ridge 
Oral  Cavity 


Village  of  Lower  Ja 


Fig.  310. — Vertical   TransV-ERsk   .Skciiun   iiirough    Head  of  Human    Embryo, 

ABOUT  THE   TWELFTH    WEEK,  SHOWING  THE   SINGLE    BUCCAL  CavitY   TRANSFORMED   INTO 

THE  Oral  and  Nasal  Cavities.     X  3°- 


the  most  part  made  up  ot  connective-tissue  cells,  about  isolated 
bundles  of  which  osteoblasts  arrange  themselves,  resulting  in 
the  production  of  two  intermembranous  bony  plates. 

By  the  fourteenth  week  a  further  advance  in  the  generation 
ot  the  hard  palate  is  noted,  the  septum  now  being  largely  com- 
posed ot  calcified  tissue.  The  disposition  for  these  primitive 
bony  plates  to  exist  as  separate  and  distinct  processes  is  ex- 
emplified at  the  median  line  by  a  definite  separation  formed 
by  the   connective-tissue  sheath   from   which   they  are   derived. 


46o  EMBRYOLOGY. 

Covering-  the  surface  of  the  hard  palate  there  now  appears  a 
thin  layer  of  mucous  membrane. 

The  Floor  of  the  Mouth. — Having  thus  briefly  noted  the 
evolution  of  the  roof  of  the  mouth,  let  us  next  consider  the  floor 
of  the  cavity,  the  tongue  and  its  attached  muscles,  together 
with  considerable  glandular  tissue  making  up  the  bulk  of  this 


Fig.  311. — Section  through  Base  uf  Tongue  and  Lower  Jaw.     x  4°- 

district.  Figure  311  is  a  vertical  transverse  section  through  the 
floor  of  the  mouth  about  the  tenth  week  in  the  human  fetus,  or 
at  a  period  somewhat  later  than  that  shown  in  the  previous 
illustration.  An  examination  of  the  parts  in  general  at  a  time 
prior  to  this  is  of  little  value,  save  the  early  preparation  for  the 
development  of  the  teeth,  which  will  be  referred  to  later.  The 
tissues  and  organs  here  shown  will  be  recogfnized  as  the  tongue 
(A),  the  glandular  tissues  (B),  the  forming  jaw  [C),  with  devel- 
oping tooth-germs  at  D  D. 


THE   FLOOR   OF   THE   MOUTH, 


461 


The  tongue  appears  on  the  floor  of  the  mouth  between  the 
thirtieth  and  thirty-sixth  days  as  a  bud  from  the  mesoblast  cov- 
ered by  a  layer  of  cells  of  epiblastic  origin.  The  muscle-fibers, 
be  they  intrinsic  or  extrinsic,  are  all  of  the  striated  variety.  In  a 
very  short  time,  and  at  a  comparatively  early  period,  the  tongue 
becomes  an  independent  organ,  presenting  most  of  the  char- 
acteristics common  to  it  after  birth.  Not  a  small  portion  of  the 
floor  of  the  mouth  is  made  up  of  another  class  of  tissue  which, 


g^       ^^^%ja 

|S 

}   ■ 
> 

\  ■■ 

H 

1^  "^^^^^ 

Sl"' 

s 

^ta 

^w 

Mi 

w 

Fig.  S12. — Section  of  Sublingual  District. 


although  eventually  a  distinct  organism,  is  composed  almost 
entirely  of  epithelium.  These  cells,  together  with  the  connec- 
tive-tissue cells,  and  eventually  blood-vessels,  unite  in  the  pro- 
duction of  a  true  salivary  gland,  the  sublingual.  Figure  312 
shows  the  early  character  of  the  tissue,  together  with  its  relation 
to  surrounding  parts.  The  section  is  one  from  the  region  of 
the  premolars,  and  is  bounded  above  by  the  tongue,  laterally 


462  EMBRYOLOGY. 

by  the  borders  of  the  jaw,  and  below  by  fibers  of  the  mylo- 
hyoid and  digastric  muscles.  Three  distinct,  lobes  or  sections 
of  the  gland  are  observed,  the  two  largest  being  separated  by  a 
reticular  network  of  connective  tissue. 

The  general  character  of  these  developing  glands  even  at  this 
early  period  (about  the  twelfth  week)  appears  to  be  very  similar 
to  the  matured  organ,  being  composed  of  a  number  of  small 
tubes  emptying  into  a  single  duct,  constituting  a  gland  of  the 
compound  tubular  variety. 

Let  us  next  give  some  consideration  to  the  embryology  of  the 
mouth  In  its  entirety  ;  and  to  do  this,  it  is  necessary  to  make  sec- 
tions of  the  parts  in  various  directions. 

The  growth  of  the  cavity  is  usually  studied,  and  probably  to 
the  best  advantage,  by  vertical  transverse  sections,  and  attention 
will  first  be  called  to  a  number  of  sections  made  in  this  way^ 
beginning  at  the  lips  and  passing  backward  through  the  incisor 
region,  and  finally  through  the  districts  occupied  respectively 
by  the  cuspids  and  molars.  The  period  at  which  such  an  in- 
vestigation Is  made  has  much  to  do  with  the  character  of  the 
tissue  involved,  but  the  time  best  suited  to  the  purpose  is  in- 
cluded between  the  fortieth  and  sixtieth  days.  At  this  time 
nearly  all  the  tissues  making  up  the  organs  and  parts  which 
enter  into  the  construction  of  the  cavity  have  advanced  to  such 
a  degree  of  perfection  that  the  investigation  may  proceed  with 
considerable  satisfaction. 

Figure  313  shows  a  cross-section  through  one  of  the  primitive 
labial  folds  about  the  period  named.  Little  is  to  be  observed  in 
this  district  at  this  early  period  except  the  simple  cells  of  three 
varieties  which  serve  to  make  up  the  parts,  but  attention  is  at 
once  attracted  to  the  abundant  thickness  of  the  epithelium  given 
to  the  lip. 

If  a  section  be  made  somewhat  to  the  distal  of  that  previously 
shown,  a  marked  change  in  the  relationship  existing  between 
the  various  cell  layers  is  observed  in  a  body  of  cells  of  another 
character,  those  which  are  destined  to  become  the  cartilage  of 
Meckel,  and  about  which  the  younger  layer  of  cells  of  mucous 
membrane  are  observed  outlining  a  new  district. 

If  a  section  be  made  through  this  same  location,  say  about  the 


THE    FLOOR   OF   THE   MOUTH. 


463 


forty-eighth  day,  a  vast  change  in  the  appearance  of  the  parts  is 
noticed  (Fig.  314).  The  buccal  walls  of  the  mouth  have  in  a 
measure  become  complete  by  a  union  of  the  upper  and  lower 
sections,  the  union  at  this  time  being  accomplished  through  the 
agency  of  the  embryonal  epithelium.  A  cartilaginous  nasal 
septum  has  made  its  appearance,  and  active  preparation  for  the 
ossification  of  the  maxillae  is  apparent.     In  the  center  of  the  sec- 


FiG.  SI  V — -Embryonal  Labial  Mucous  Membrane. 


tion  is  a  distinct  body  ot  cells  forming  Meckel's  cartilage,  and 
early  preparations  for  the  growth  of  the  teeth  may  be  seen  at  a 
by  a  dipping  down  of  the  surface  epithelium. 

A  transverse  section  through  the  same  district  about  the  six- 
tieth day  (Fig.  315)  shows  all  the  parts  strongly  differentiated. 
Calcification  has  taken  place  to  a  considerable  extent  in  the  lower 
jaw,  the  two  halves  being  at  this  period,  and  for  some  months 


464  EMBRYOLOGY; 

afterward,  separate  and  distinct.  Many  muscle  bundles  are 
observed  beneath  the  jaw,  and  beyond  these  the  integument 
with  its  numerous  blood-vessels  and  nerves,  most  of  which  are 
seen  in  transverse  section.  A  cross-section  upon  the  same  subject 
about  the  sixtieth  day  in  the  region  of  the  cuspids  finds  the  tissues 
and  organs  advanced  to  a  certain  degree  of  perfection.  (See  Fig. 
311.)     The  tooth-germs  of  the  cuspid  teeth  have  their  crowns  out- 


FiG.  314. — Section  through  Base  of  Jaw. 

lined  by  the  cells  composing  them  ;  the  tongue  with  its  complex 
muscular  arrangement  has  become  a  specialized  and  independent 
organ,  while  beneath  it  we  see  that  product  of  the  epiblast,  the 
glandular  structure,  so  plentifully  supplied  to  the  floor  of  the 
mouth  in  this  locality. 

Passing    further    back    into    the    region  of  the    molars,    the 
appearance  of  the  parts  does  not  differ  to  any  marked  degree 


THE   FLOOR   OF  THE  MOUTH. 


465 


from  that  in  the  cuspid  district,  except  in  the  general  distribu- 
tion of  the  muscular  fibers  of  the  tongue,  and  the  appearance 
of  the  submaxillary  gland,  here  appearing  in  three  distinct  lobes 
or  parts. 

It  has  been  previously  stated  that  sections  made  in  the  direc- 
tion of  those  already  considered  are  usually  employed  to  study 
these  parts,  but  much  is  to  be  gained  by  supplementing  these 
with  sections  made  in  other  directions. 


Fig.  315. — Section  through  the  Wall  of  the   Mouth   of  an   Embryo,  Sixtieth 

Day. 


Figure  316  shows  a  longitudinal  section,  or  one  made  from 
mesial  to  distal  through  the  lower  jaw  at  or  near  the  median 
line,  the  parts  included  within  the  field  being  the  labial  folds  at 
A,  the  mandible  at  C,  the  tongue  at  D,  and  a  tooth-germ  at  E. 
An  examination  of  the  lips  shows  them  to  be  covered  with  a 
varying  thickness  of  embryonal  epithelial  cells  which  are  con- 


466 


EMBRYOLOGY. 


tinued  backward  over  the  future  alveolar  ridge  and  thence  to 
the  hard  palate  above,  or  over  the  floor  of  the  mouth  and  the 
surface  of  the  tongue  below. 

Meckel's  Cartilage. — One  of  the  earliest  products  of  the 
mesoblast  is  that  which  results  in  the  production  of  Meckel's 
cartilage,  which  is  closely  associated  with  the  growth  and  early 


e — iH 


Fig.  316. — Longitudinal  Section  through  Chin  of  Emkryo  Lame. 


support  ot  the  lower  jaw.  In  the  beginning,  as  already  pointed 
out,  the  mandibular  and  hyoid  arches  resemble  one  another,  but 
soon  after  they  become  fully  established  they  take  on  different 
functions,  and  with  this  become  dissimilar.  The  first  appear- 
ance of  this  cartilage  as  a  distinct  body  of  cells  is  found  about 


MECKEL'S   CARTILAGE. 


467 


the  middle  of  die  second  month,  and  when  a  transverse  section 
of  the  jaw  is  made  for  the  purpose  of  studying  its  location  and 
environments  (see  Fig  317),  it  is  found  near  the  base  of  the  fetal 
head,  considerably  below  and  to  the  outside  of  the  base  of  the 
tongue.  At  mid-jaw  it  appears  as  a  circular  body  of  cells  sepa- 
rated from  the  surrounding  parts  by  a  distinct  layer  of  elongated 


Fig.  317.— Section  through  Base  of  Lower  J.wv,  showing  Meckel's  Cartilage. 


cells.  Even  at  this  early  period  a  portion  of  the  bony  structure 
of  the  jaw  is  outlined  by  an  aggregation  of  connective-tissue 
cells,  and  the  forming  cartilage  appears  to  subserve  the  purpose 
of  controlling  the  outline  of  the  future  jaw.  The  bow-shape  of 
the  cartilage  is  manifest  as  we  pass  toward  the  symphysis  by 


468  EMBRYOLOGY. 

the  lateral  halves  approaching  each  other  (Fig.  31 8),  but  the  cir- 
cular character  of  the  cartilage  in  cross-section  is  still  retained. 
Figure  319  represents  a  section  through  the  symphysis  about 
the  eighteenth  week,  and  shows  the  two  halves  of  the  cartilage 
closely  associated,  but  not  united,  the  separation  being  by  a 
layer  of  connective-tissue  cells  passing  between  the  two.  It 
will  be  noted  also  that  the  cartilaee,  instead  of  belne  near  the 


Fig.  318. — Section  through  Lower  Jaw.     M.  C.   Meckel's  cartili 


base  of  the  jaw  as  in  figure  318,  now  appears  near  the  floor  of 
the  mouth. 

Figure  320  shows  the  relations  existing  between  the  two 
halves  of  Meckel's  cartilage  and  the  growing  mandible  at  the 
median  line  [a).  It  also  illustrates  how  little  the  development 
of  the  bone  is  dependent  upon  the  cartilage,  the  growth  ot 
the  former  being  in  this  district  far  below   and   apparently  dis- 


MECKEL'S   CARTILAGE. 


469 


tinct  from  the  latter.  Here,  as  in  the  upper  jaw,  the  periosteal 
cells  from  either  side  are  observed  to  unite  at  the  symphysis  and 
pass  as  a  somewhat  thickened  layer  between  the  two  bones,  the 
only  difference  in  the  final  change  which  takes  place  between 
the  two  being  that  in  the  upper  jaw  a  suture  results,  while 
in  the  lower  jaw  a  layer  of  solid  bone  is  formed.  The  charac- 
ter of  this  cartilaginous  framework  as  well  as  the  cells  which 


Fig.  319. — Meckel's  Cartilage  {M.  C.)  at  the  Sy.mi'hysis. 


divide  the  two  halves  is  shown  in  figure  321,  the  cardlage  cells 
being  oblong  or  cylindrical,  with  a  bountiful  supply  of  intercel- 
lular substance,  while  the  connective-tissue  cells  are  oblong  or 
spindle-shaped. 

As  soon  as  ossification  in  the  jaw  takes  place  to  any  extent, 
the  cartilage  begins  to  atrophy,  that  portion  lying  next  to  the 
jaw  deo-eneratino-  first,  so  that  by  the  tenth  or  twelfth  week  it 


EMBRYOLOGY. 


has  entirely  disappeared,  but  before  this  takes  place  we  find  it 
surrounded  by  the  periosteum,  and  finally  completely  inclosed 
within  the  bone. 

Figure  322  shows  the  character  of  the  cartilage  cells  about 
the  time  that  they  are  beginning  to  atrophy.  It  will  be  observed 
that  the  cells  are  inclined  to  a  change  in  form,  and  that  they  are 


Fig.  320. — Ossification  of  the  Mandible  at  the  Median  Line. 

proportionately  larger  with  large  nuclei  and  nucleoli.  A  repre- 
sents the  district  nearest  the  jaw,  and  the  cells  in  this  region 
have  already  lost  their  characteristic  outline. 

Further  Consideration  on  the  Subject  of  Tooth  Devel- 
opment.— One  phase  of  the  subject  to  which  special  attention 
will  be  given  in  this  chapter  is  that  which  denotes  the  period  at 


Fig.  321. — Section  of  Meckel's  Cartilage  at  Median  Line. 


Fig.  322. 


472  EMBRYOLOGY. 

which  the  various  events  take  place.  This  is  a  part  of  the  study 
which  is  very  difficult  to  determine,  and  it  would  appear,  for  this 
reason,  if  for  no  other,  that  investigators  of  recent  years  have 
been  perfectly  satisfied  to  accept  the  results  arrived  at  by  their 
predecessors  without  any  apparent  effort  to  qualify  the  deduc- 


FiG.  32J. — Transverse  Section  through  Primitive  Jaws  of  Human  Fetus. 


tions.  It  has  for  a  long-  time  been  conceded  that  the  primitive 
changes  which  ultimately  result  in  the  formation  of  a  tooth-germ 
are  first  noted  in  a  heaping  up  of  the  epithelial  cells  over  the 
district  representing  the  surface  of  the  future  jaw.  While  in 
many  instances  this  is  true,  there  are  reasons  why  it  cannot  be 
considered  an  essential  feature.     In  the  first  place,  such  a  con- 


TOOTH   DEVELOPMENT. 


dition  is  not  always  present,  as  shown  in  figure  323,  a  transverse 
section  through  the  primitive  jaw  of  a  human  fetus  about  the 
fortieth  day.  The  tooth  bands  at  A  and  B  have  penetrated  the 
submucous  tissue  for  a  considerable  depth,  but  the  surface  epi- 
theHum  does  not  show  a  greater  thickness  at  these  points  than 
it  does  over  the  general  surface  of  the  cavity. 

Figure  324  shows  a  section  made  in  the  same  direction  upon 


Fig.  324. — Transverse  Section  through  Lower  Jaw,  Human  EiMURYO,  Sixtieth 
Day. 


a  human  embryo  about  the  sixtieth  day,  and  while  the  tooth 
band  {A)  has  penetrated  the  embryonal  connective  tissue  to  a 
greater  depth,  there  is  yet  no  increase  in  the  thickness  of  the 
epithelium,  but  rather  a  disposition  for  the  parts  to  become 
depressed.  Another  reason  why  the  heaping  up  of  the  super- 
ficial layer  of  cells  forming  the  embryonal   mucous  membrane 


474  EMBRYOLOGY. 

should  not  be  considered  the  first  sign  of  the  preparation  for 
tooth  development  lies  in  the  fact  that  these  cells  are  not 
directly  interested  in  the  process,  but  that  the  inflection  of  cells 
which  results  in  the  formation  of  the  tooth  band  results  from 
the  deep  or  infant  layer  of  cells  known  as  Malpighi's  layer,  as 
shown  at  B  (Fig.  324). 


Fig.  325. — Section  through  Tooth  Band  of  Hum.-vn  Embryo. 

There  is  no  question  but  that  the  location  from  which  the  sec- 
tion is  taken  has  much  to  do  with  the  character  and  thickness 
of  the  older  layer  of  epithelial  cells,  and  that  they  do  at  certain 
points  constitute  an  epithelium  exceeding  in  thickness  that  of 
other  parts  of  the  cavity,  but  this  condition  most  frequently 
occurs  after  the  enamel  organ  has  assumed  definite  proportions. 

After  the  formation  of  the  tooth  band,  which,  it  rhust  be 
remembered,  encircles  the  entire  jaw  in  the  form  of  a  well- 
defined  body  of  oval  epithelial  cells  from  the  infant  layer  (shown 


TOOTH    DEVELOPMENT,  475 

at  A  in  cross-section,  Fig.  325),  the  next  step  in  the  process  is 
one  which  concerns  the  location  for  the  individual  buds  for  the 
enamel  organs  of  the  various  teeth,  and  the  approximate  time 
at  which  these  appear. 

In  the  human  subject  we  find  ten  such  spots  appearing  upon 
the  lamina  given  off  from  the  lingual  face  of  the  tooth  band. 
These  do  not  appear,  however,  at  the  free  extremity  of  the  band. 


Fig.    326. — ToOTH-GERM,    EMHRYO    L.'VMI!,    CoRKESl'ONDING   TO    SIXTIETH     DAY    (HUMAN). 

X  4°- 


but  at  some  little  distance  toward  the  surface  from  this  point,  as 
shown  in  figure  326.  In  this  section  the  tooth-germ  is  severed 
from  the  surface  epithelium,  but  this  is  not  a  true  condition  at 
this  period,  as  it  still  retains  its  connection  with  the  surface  by  a 
narrow  band  of  cells,  the  neck  of  the  enamel  organ.  Two  dis- 
tinct classes  of  cells  are  now  (sixtieth  day)  concerned  in  the 
process  of  tooth  development,   those  at   A  being  of  epithelial 


476  EMBRYOLOGY, 

origin  and  forming  the  future  enamel  organ,  while  at  B  an  aggre- 
gation of  cells  from  the  mesoblast  provides  for  the  generation  of 
the  pulp  and  dentin.  At  C  the  narrow  band  of  cells  which  should 
continue  to  the  surface  is  shown,  while  the  free  extremity  of 
the  same  body  of  cells  at  D  will  persist  and  eventually  become 
the  germ  for  the  succeeding  tooth.  In  regard  to  the  time  at 
which  the  buds  for  the  various  teeth  appear,  it  might  be  ex- 
pected that  the  same  variation  which  follows  the  development 
and  erupdon  of  the  teeth  throughout  would  obtain,  but  such  is 
not  the  case,  the  buds  for  the  deciduous  incisors  appearing 
about  the  sixtieth  day,  while  the  germs  for  their  permanent  suc- 
cessors are  but  httle  later  in  forming. 

By  referring  to  figure  316,  which  shows  a  longitudinal  section 
through  the  lower  jaw  about  the  twelfth  week,  the  deciduous 
incisor  is  seen  well  outlined  by  its  formative  cells,  while  imme- 
diately to  the  lingual  appears  a  section  of  the  germ  for  the  per- 
manent cuspid.  Notwithstanding  this,  there  elapses  a  period  of 
several  years  between  the  eruption  of  these  teeth.  The  same 
relative  progress  will  be  noted  between  the  first  and  second 
teeth,  be  the  subject  human  or  otherwise  ;  requiring  many  years 
to  complete  dentition,  in  the  former,  while  in  most  of  the  lower 
animals  the  same  process  occupies  but  a  comparatively  short 
time. 

The  next  stage  in  the  development  of  a  tooth  to  which  atten- 
tion will  be  called  is  that  in  which  the  entire  tooth-crown  is  out- 
lined by  the  dentin  papilla  and  surrounded  by  its  epithelial  cap, 
the  enamel  organ.  Such  an  advance  in  the  process  is  shown 
in  figure  327,  together  with  the  surrounding  structure.  When 
this  stage  is  reached,  the  individual  cells  of  the  tooth-germ  are 
strongly  differentiated,  and  the  odontoblasts  are  making  their 
appearance  about  the  summits  of  the  cusps. 

Up  to  this  time  the  cells  present  are  those  which  result  in  the 
formation  of  but  tWo  of  the  calcified  tooth  tissues,  but  now  there 
is  a  marked  disposition  upon  the  part  of  the  periosteum  of  the 
jaw  to  pass  down  by  the  side  of  the  enamel  organ  {A),  this  being 
the  first  indication  of  the  formation  of  the  tooth  follicle,  the 
alveolodental  membrane,  and  cementum.  At  this  period  it  will  be 
observed  that  there  appears  in  this,  the  molar  region,  a  "heap- 


TOOTH    DEVELOPxMENT.  477 

ing  up  "  not  only  of  the  surface  epithelium,  but  also  of  the  con- 
nective tissue  as  well. 

Figure  328  shows  a  section  through  the  growing  mandible 
taken  from  a  portion  of  the  tissue  not  occupied  by  a  tooth-germ. 
This  is  of  interest,  first,  as  giving  a  view  of  the  detached  tooth 
band  in  cross-section,  at  A ;  second,  by  showing  the  distribution 
of  the  periosteum  to  the  interior  of  the  jaw  to  serve  the  double 


27. ^Tooth-germ,  Premolar,   Emuryo  Lamb.     X  4°- 


function  of  the  future  tooth  sac  and  alveolodental  membrane;  and, 
third,  the  thickened  epithelium  with  the  underlying  tissue  push- 
ing into  it. 

Although  the  germ  for  the  second  tooth  may  be  observed  at 
a  period  somewhat  prior  to  this,  a  study  of  some  of  its  charac- 
teristics is  best  made  at  this  time.  The  fact  has  already  been 
referred  to  that  this  interesting  phenomenon  occurs  soon  after  or 


478 


EMBRYOLOGY. 


even  simultaneously  with  that  for  the  first  tooth,  a  portion  of  the 
primitive  cord  for  the  latter  persisting  as  the  germ  for  the 
former. 

In  figure  329  the  cells  forming  the  primitive  germ  for  the 
enamel  organ  of  one  of  the  permanent  teeth  are  shown  highly 
magnified.       It    will    be    observed    that    they  are    of    the  sim- 


FiG.  328. — Section  throuch  J.wv  of  Embryo  Lamb,  in  a  District  not  Occupied  by 
A  Tooth-germ.     X  4°- 


plest  epithelial  character,  and  that  they  are  derived  directly 
from  the  enamel  organ  of  the  pre-existing  tooth,  on  the  one 
hand,  while,  on  the  other,  they  communicate  with  the  surface  by 
a  narrow  band  of  cells.  In  this  way  it  is  for  a  time  dependent 
upon  both  of  these  parts  for  continuance  and  growth,  but  after 


TOOTH    DEVELOPMENT.  479 

a  time  it,  too,  like  its  predecessor,  severs  its  connection  with  the 
surface,  but  remains  intact  with  the  epithelial  cells  of  the  former 
enamel  organ  until  these  cells  begin  to  atrophy. 

The  cells  which  make  up  this  primitive  germ  are  of  three 
varieties  :  the  inner  layer,  or  those  derived  from  the  epithelium 
of  the  enamel  organ  of  the  first  tooth,  being  small  and  spheroidal ; 
those  of  the  outer  layer,  which  spring  from  the  surface  epithe- 


FiG.  329. — Primitive  Bud  for  Enamel  Organ  of  Permanent  Tooth,  Human 
Embryo.     X  lOo. 


lium,  being  proportionately  larger  and  cylindrical  or  oblong; 
while  those  which  intervene  are  markedly  irregular  in  outline. 
In  this  respect — that  is,  in  the  character  of  the  early  cell  layers 
— the  tooth-germs  for  the  permanent  teeth  differ  from  those  of 
the  deciduous. 

The  question  of  the  origin  of  those  teeth  which  have  no  pre- 
decessors is  one  upon  which  there  has  always  been  more  or  less 
discussion,    some    writers    contending   that    they   are    derived 


48o  EMBRYOLOGY. 

directly  from  the  oral  epithelium  by  a  special  generation  of  cells 
for  each  tooth,  while  others  are  of  the  opinion  that  as  the  jaw 
grows  backward  certain  changes  take  place  which  result  in  the 
establishment  of  an  epithelial  fold  or  lamina,  in  every  particular 
corresponding  to  the  tooth  band  of  the  deciduous  teeth.  With 
these  two  conflicting  opinions  in  mind,  a  number  of  sections  were 
made  through  the  extreme  distal  end  of  the  jaw.  The  result 
favored  the  latter  theory,  for  here  the  tooth  band  is  seen  similar 
in  form  and  location  to  that  observed  in  the  jaw  in  those  locations 
from  which  succedaneous  teeth  result. 


CHAPTER  VII. 


ANOMALIES  OF  THE  TEETH. 


It  has  always  been  conceded  that  the  dental  organs  of  man  are 
susceptible  of  much  variation  in  form  and  structural  arrange- 
ment, and  that  frequently  this  variation  is  so  positive  that  the 
organ  is  pronounced  anomalous  in  character.  Just  where  the 
line  of  distinction  between  the  normal  and  abnormal  should  be 
drawn  is  a  subject  worthy  of  some  consideration.  Some  au- 
thorities define  the  word  anomaly  as  a  marked  deviation  from 
the  normal,  while,  in  the  opinion  of  others,  a  much  broader 
meanino^  is  accorded  it;  and  we  find  all  those  conditions  which 


Fig.  330. — Anomalous  Teeth. 


are  in  themselves  an  irregularity  from  the  typical  structure  or 
occurrence  included  in  this  category.  Under  the  first  definition 
a  given  structure  or  organ  is  accorded  a  wide  field  ior  its  normal 
existence,  while  under  the  latter  but  slight  deviation  is  necessary 
to  classify  it  among  the  abnormal. 

Upon  first  thought  it  would  appear  that  the  ability  or  inability 
of  a  tissue  or  an  organ  to  perform  its  special  function  should,  in 
a  measure,  decide  the  question  of  the  nature  of  its  being,  and 
no  doubt  to  a  certain  extent  this  is  true  ;  but  while  the  action  of 
an  organ  or  a  part  of  the  body  may,  by  observation,  appear  en- 
tirely satisfactory,  it  is  only  so  at  the  expense  of  other  organs 
31  4S1 


482  ANOMALIES. 

or  tissues,  and  these  in  the  course  of  time,  by  this  extra  exer- 
tion, become  hypertrophied  or  in  other  ways  pathologic. 

While  this  is  especially  applicable  to  those  organs  or  tissues 
which  have  a  wide  range  of  function,  it  may  with  a  good  deal  of 
force  be  applied  to  the  dental  organs  and  their  immediate  envi- 
ronments. Anomalous  conditions  in  the  teeth  may  originate 
in,  or  be  confined  to,  one  or  more  of  the  tooth  tissues,  in  any 
of  which  the  structural  disarrangement  may  eventually  result 
in  the  death  or  degeneracy  of  the  part.  Enamel  malformation 
is  of  such  a  character  that  it  may  be  observed  upon  the  surface 
either  in  the  form  of  a  multiplication  of  cusps,  or  by  an  extra 
development  of  the  various  ridges  formed  by  pronounced  folds 
of  this  tissue.  But  probably  the  most  disastrous  anomaly  of 
the  enamel,  and  one  frequently  responsible  for  the  downfall  of 
this  tissue,  is  found  in  some  defect  of  its  structural  arrangement 
other  than  those  just  referred  to.  In  some  instances  the  enamel 
rods  of  a  given  district,  instead  of  being  normally  distributed 
by  assuming  a  direction  principally  at  right  angles  to  the  long 
axis  of  the  tooth  crown,  are  arranged  without  regard  to  the  base 
or  periphery  of  the  tissue,  'and  we  have  as  a  result  an  anomaly 
of  structure.  The  question  of  normal  and  abnormal  rod 
distribution  now  presents  itself,  because  in  certain  locations 
— ?'.  e.,  the  summits  of  the  cusps — an  arrangement  of  the  rods 
similar  to  that  referred  to  is  so  common  that  it  may  be  con- 
sidered a  normal  condition,  while,  if  a  like  distribution  was 
found  in  other  locations,  the  tissues  should  properly  be  con- 
sidered abnormal. 

Malformed  teeth,  in  respect  to  the  number  and  forms  of  the 
cusps  present,  are  not  alone  confined  to  the  enamel,  but  also  to 
the  dentin  which  first  records  the  tooth  form  on  its  periphery. 

Anomalies  in  the  general  contour  of  the  tooth  crown  are 
usually  confined  to  the  incisors  and  third  molars,  both  the  dentin 
and  the  enamel  contributing  to  the  deformity.  Here  the  defect 
is  usually  so  pronounced  that  but  little  difficulty  is  experienced  in 
properly  classifying  the  organ.  One  of  the  most  frequent  varia- 
tions in  form  met  with  in  these  locations  is  found  in  the  peg- 
shaped  or  cone-shaped  crown.  If  it  were  possible  it  would  be  in- 
teresting to  trace  the  development  of  Such  a  malformation  ;  but 


LACK  OF  DENTITION.  483 

with  our  present  knowledge  of  this  process  in  general,  there  is 
little  doubt  as  to  its  origin,  the  enamel  organ  failing  to  fulfil  its 
early  and  primary  function  of  molding  the  tooth  crown  in  the 
dentin  papilla,  the  responsibility  for  this  resting  in  the  special 
cells  composing  it,  as  well  as  the  so-called  stellate  reticulum, 
which,  it  is  believed,  exerts  a  controlling  influence  over  the  form 
of  the  enamel  cap. 

While  the  organic  defects  of  tooth  crowns  are  numerous  and 
varied,  those  which  are  confined  to  the  roots  are  most  frequent, 
in  many  instances  interfering  to  some  extent  with  the  function 
of  the  organ.  When  a  given  peculiarity  is  confined  to  this  por- 
tion of  the  tooth,  it  is  frequently  difificult  to  discriminate  between 
the  normal  and  the  abnormal.  Certain  teeth  are  recognized  as 
normal  when  either  a  single  root  or  two  roots  are  present,  and 
the  acceptance  of  this  fact  increases  the  difficulty  of  a  proper 
classification  of  its  peculiarities. 

In  very  rare  instances  do  we  find  the  roots  of  the  cuspidate 
teeth  more  or  less  crooked  ;  yet,  at  the  same  time,  many  deci- 
dedly crooked  roots  are  considered  within  the  natural  law ; 
while,  on  the  other  hand,  roots  with  but  little  more  deflection  are 
classed  as  anomalous. 

Marked  flexions  of  roots  or  crowns,  cases  of  fusion  or  con- 
crescence, are  usually  so  positive  in  character  that  an  anomalous 
condition  is  at  once  acknowledged.  While  tooth  anomalies  are 
usually  referred  to  as  external,  or  as  belonging  to  the  hard 
tissues  of  the  organ,  they  are  not  infrequently  found  in  the  pulp 
or  pulp  cavity.  This  cavity,  normally  following  the  external  con- 
tour of  the  tooth,  is  subject  to  much  variation  in  outline  and 
capacity,  regardless  of  those  changes  which  are  incident  to  the 
continuous  process  of  dentinification. 

Among  these  are  a  complete  division  of  the  pulp  chamber ; 
horn-like  processes  penetrating  the  dentin  in  the  direction  of  the 
occlusal  surface  in  locations  where  they  would  be  least  expected  ; 
an  unusual  number,  or  a  peculiar  distribution  of  the  canals,  etc. 

Lack  of  Dentition. — Cases  in  which  there  is  a  total  absence 
of  teeth  have  been  reported.  Guilford  reports  the  case  of  a  man 
fifty  years  of  age  who  never  had  teeth,  the  jaws  not  differing  in 
appearance  or  form  from  those  of  a  person  whose  teeth  have 


484  ANOMALIES. 

been  extracted.  The  mother  of  the  subject  had  the  usual 
number  of  teeth,  but  the  grandmother  and  an  uncle  were  both 
edentulous  and  hairless  from  birth.  J.  Tomes  mentions  two 
cases  having  been  reported  to  him,  and  Linderer  mentions  one. 

On  the  other  hand,  instances  of  a  third  dentition  have  been 
reported,  but  there  is  great  possibility  of  such  observation  being 
erroneous.  Teeth  belonging  to  the  permanent  set  which  may 
have  remained  unerupted  for  years  could  readily  be  mistaken 
as  predecessors  of  a  third  set,  when  in  after  years  they  make 
their  appearance. 

Supernumerary  Teeth. — All  teeth  appearing  in  the  mouth 
in  addition  to  the  normal  number  are  designated  as  super- 
numerary teeth.  These  are  divided  into  two  classes,  those 
normal  in  size  and  form  and  those  abnormal  in  size  and  form. 
The  first-named  are  most  likely  to  be  of  the  simple  class,  in- 
cisors and  cuspids,  and  they  may  occupy  a  regular  position  in 
the  arch  or  may  be  found  inside  the  arch  closely  associated  with 
teeth  of  the  same  type.  Supernumerary  bicuspids  and  molars 
are  sometimes  present  either  in  regular  position  in  the  arch  or 
inside  of  it.  When  the  jaws  are  long  enough  to  accommodate 
them,  an  additional  molar  may  appear  back  of  the  third  molar, 
making  four  molars  instead  of  three,  and  cases  in  which  two 
extra  molars  were  thus  placed  have  occasionally  been  met  with. 

Supernumerary  teeth  of  the  second  class  or  those  which  are 
abnormal  in  size  and  form  are  usually  inclined  to  be  cone-shaped 
and  small  in  size,  this  in  respect  to  the  root  as  well  as  the  crown. 
Supernumerary  teeth  of  this  character  are  usually  found  in  the 
incisor  region,  but  it  occasionally  happens  that  they  are  found 
in  the  molar  district,  but  here,  instead  of  having  a  single  cone 
for  the  crown,  they  are  mostly  made  up  of  a  number  of  smaller 
cones  resembling  many  small  cusps  on  the  occlusal  surface. 
The  number  of  supernumerary  teeth  may  vary  from  one  to  eight 
or  ten.  In  the  latter  instance  they  are  usually  scattered  through 
the  entire  alveolar  border  of  the  hard  palate.  As  many  as  ten 
or  twelve  teeth  thus  located  have  been  reported. 

Again,  certain  teeth  are  frequently  missing  from  the  arch. 
This  may  be  occasioned  by  delayed  eruption,  or  it  may  be 
the    result  of  improper  activity  within    the  tooth-germ    itself, 


SUPERNUMERARY  TEETH.  485 

SO  that  the  tooth  may  have  failed  to  develop.  The  teeth 
most  frequently  missing  are  the  upper  lateral  incisors.  The 
probable  reason  for  this  lies  in  the  fact  that  the  germs  for 
these  teeth  are  located  very  near  the  surface  of  the  bone, 
and  in  some  instances  they  are  not  even  protected  by  a 
thin  layer  of  bone  over  the  follicle.  Being  thus  situated,  they 
are  more  or  less  exposed  to  violence  sufficient  to  destroy  the 
germs  and  thus  render  development  of  the  teeth  impossible. 
Lack  of  certain  teeth  in  the  mouth  appears  to  be  to  some 
extent  an  hereditary  feature,  the  condition  being  transmitted 
from  parent  to  child.  Anomalies  as  to  the  size  of  individual 
teeth  are  frequently  noticed.  When  this  is  the  case,  it  does  not 
usually  include  the  entire  set,  but  is  confined  to  one  or  two, 
usually  to  the  same  teeth  on  each  side.  Teeth  that  are  above 
the  usual  size  are  generally  found  in  persons  of  large  build,  but 
if  all  the  teeth  are  proportionate  in  size  they  can  not  be  included 
within  the  abnormal  class.  The  upper  incisors  are  most  fre- 
quently affected  in  this  way.  Cases  have  been  reported  in 
which  the  central  incisors  in  the  upper  jaw  have  been  fully  twice 
the  size  which  they  should  normally  have  been,  and  all  the  re- 
maining teeth  in  the  mouth  perfect  as  to  shape  and  size.  Ac- 
companying the  abnormal  condition  it  is  usual  to  find  the  teeth 
thus  affected  more  or  less  abnormal  in  outline,  but,  notwithstand- 
ing this,  retaining  their  form  sufficiently  well  to  permit  their 
proper  classification. 

There  are  likewise  teeth  that  are  deficient  in  size.  This  does 
not  refer  to  teeth  all  relatively  small,  as  frequently  found  in  per- 
sons of  small  frame  and  stature.  This  anomalous  condition  is 
often  present  in  the  upper  lateral  incisors  and  in  the  third 
molars.  Teeth  that  are  deficient  in  size  generally  possess  their 
normal  shape. 

While  the  crowns  of  the  teeth  are  susceptible  to  the  above 
variations,  the  roots  appear  to  be  anomalous  much  more 
frequently  and  to  a  more  marked  extent  than  are  the  crowns. 
Most  important  among  these  may  be  mentioned  flexions  of 
the  roots.  Curvatures  in  the  roots  may  be  found  either  in 
single-rooted  teeth  or  in  multirooted  teeth,  and  the  point  of 
flexion    may  be  located   either   at    the  center    of  the  root   or 


486  ANOMALIES. 

near  its  apex,  or  both  of  these  points  may  be  affected. 
Flexions  in  the  roots  of  the  teeth  present  a  great  variety 
in  form.  A  single  curve  in  one  direction  may  be  present 
or  a  number  of  curvatures  in  different  directions.  In  multi- 
rooted teeth  the  roots  may  be  so  flexed  that  they  will  en- 
twine about  each  other,  and  the  overlapping  portions  may  be 
or  may  not  be  fused.  Probably  the  most  important  cause  for 
the  flexions  of  the  roots  of  the  teeth  is  delayed  eruption,  this 
being  particularly  true  if  some  positive  force  prevents  the  prog- 
ress of  the  organ.  The  roots  of  the  teeth  are  frequently  anom- 
alous in  regard  to  number.  These  may  have  the  same  general 
form  and  the  same  approximate  length  as  the  normal  root  or 


Fig.  331. — Anamolous  Roots. 


roots  would  be,  but  they  are  proportionately  smaller  in  diameter. 
Cases  are  on  record  in  which  the  incisor  teeth  have  had  two  dis- 
tinct roots,  but  probably  the  most  frequent  location  for  the  mul- 
tiplicity of  roots  is  found  in  connection  with  the  third  molar. 
While  this  tooth  when  normally  developed  as  to  crown  is  usu- 
ally supported  by  three  roots,  it  sometimes  possesses  five  or  six 
smaller  roots. 

It  not  infrequently  happens  that  the  roots  of  the  teeth  are 
less  in  number  than  they  should  normally  be.  This  is  usually 
brought  about  by  the  blending  of  the  roots,  which  occurs 
during  the  developmental  stage.  This  may  be  of  two  distinct 
kinds.  In  the  first  place,  it  may  be  the  result  of  the  conversion 
of  two  or  more  pulp  canals  into  one,  or  the  individual  roots 


FUSION  AND  CONCRESCENCE. 


487 


may  be  united  by  a  body  of  cementum  being  interposed  between 
them.  In  the  former  instance  a  single  pulp  canal  is  mostly 
found  within  the  blended  roots,  while  in  the  latter  the  number 
of  canals  is  usually  normal.  With  this  blending  there  is 
generally  a  line  of  demarcation  between  the  individual  roots 
as  they  should  normally  exist,  in  the  way  of  longitudinal 
depressions    extending  through    the  entire  root  from    cervical 


Fig.  332. — Fusion  of  Molars. 


Fig.  333. — Fusion  and  Concrescence. 


line  to  apex.  This  anomaly,  like  that  of  the  multiplicity  of 
roots,  is  principally  confined  to  the  third  molars,  and  is  more 
frequent  in  the  upper  jaw  than  in  the  lower. 

Fusion  and  Concrescence. — The  union  of  two  or  more 
teeth  is  known  as  fusion  or  concrescence,  and  may  occur  either 
during  the  development  of  the  organ  or  after  this  process  has 
been  completed,     When  union  takes  place  during  development, 


ANOMALIES. 


it  is  characterized  as  fusion  ;  wlien  it  takes  place  after  the  com- 
pletion of  this  process,  it  is  known  as  concrescence.  There 
seems  to  be  but  little  doubt  that  fusion  of  the  teeth  occurs 
through  some  irregularity  in  the  tooth-germ  or  germs,  the  be- 
ginning of  the  developmental  process  taking  place  generally 
between  two  germs  and  continuing  together  until  the  complete 
calcification  of  the  organs.  Teeth  thus  united  may  have  an  in- 
ternal anatomy  corresponding  in  nearly  every  respect  to  two 
separate  teeth,  and,  on  the  other  hand,  they  may  possess  but  a 
single  pulp  chamber  and  canal.     Fusion  may  take  place  in  the 


Fig.  334.— Fusion  and  Concrescence. 


roots  of  the  teeth  alone,  when  it  is  called  partial  fiision  ;  but 
when  it  is  confined  to  the  roots  and  crowns  alike,  it  is  classified 
as  complete  fusion.  The  teeth  most  likely  to  be  affected  in  this 
way  are  the  upper  incisors  and  the  second  and  third  molars. 
Some  distinction  must  be  made  between  those  teeth  which  are 
united  by  a  layer  of  cementum,  and  those  of  true  fusion,  the 
latter  existing  only  when  there  has  been  a  union  between  dentin 
and  dentin. 

Concrescence. — In  concrescence,  the  roots  of  the  teeth  only 
can  be  affected,  as  the  union  takes  place  after  the  complete  de- 
velopment of  the  organ.  The  roots  of  one  tooth  become 
united  to  the  roots  of  another  through  an  additional  growth  of 


GEMINOUS  TEETH. 


489 


cenientum,  this  growth  being  sufficiently  extensive  to  cause  ab- 
sorption of  the  alveolar  septa  by  pressure.  Following  this 
there  is  a  resorption  of  the  pericementum.  The  cemental  tissues 
of  the  teeth  are  then  brought  in  contact,  and  coalescence  gradu- 
ally takes  place.  Concrescence  may  take  place  not  only 
between  the  roots  of  two  teeth,  but  between  the  various  roots 
of  an  individual  tooth,  this  resulting  in  the  same  manner  as 
above  described  by  the  destruction  of  the  septa  within  the  tooth 
socket.  When  concrescence  takes  place,  the  process  is  usually 
confined  to  the  apices  of  the  roots,  although  they  may  become 
coalesced  throughout  their  entire  length.*  The  teeth  most  com- 
monly affected  by  concrescence  are   the  molars  and  bicuspids. 


Fig.  335. — Geminous  Tooth. 


from  the  fact  that  the  alveolar  walls,  particularly  the  septa  about 
these  teeth  are  especially  thin,  owing  to  the  form  and  relative 
location  of  the  roots. 

Geminous  Teeth  (Fig.  335). ^It  occasionally  happens  that 
two  separate  germs  are  confined  within  a  single  sacculus,  and 
from  this  results  two  teeth,  either  similar  or  dissimilar  in  size  and 
form.  One  of  the  pair  may  be  normal  as  to  form  and  size,  while 
the  other  may  be  much  below  the  normal  size,  but  more  or  less 
perfect  in  outline.  Geminous  or  twin  teeth  may  be  united  or  en- 
tirely separate.  This  condition  is  most  frequently  found  in  the 
molar  teeth,  although  cases  in  which  the  bicuspids  and  incisors 
have  been  thus  affected  are  recorded.  Teeth  thus  formed  must 
not  be  confounded  with  those  in  which  fusion  is  the  anomaly. 


ANOMALIES. 


In  geminous  teeth  a  single  sac  contains  two  tooth-germs  from, 
which  result  two  similarly  formed  teeth  ;  in  fusion  two  follicles 
coalesce,  each  of  which  contains  its  own  eerm. 


Fig.  336. 


Fig.  337. 


Besides  the  foregoing,  the  roots  of  the  teeth  are  subject  to 
anomalies  in  size,  in  some  instances  being  abnormally  small, 
in    others    abnormally  large.     In  the  former  they  may  nearly 


GEMINOUS  TEETH.  49' 

always  be  characterized  as  anomalous,  but  this  is  not  always 
true  of  the  latter.  When  the  roots  are  abnormally  large,  it  is 
somewhat  difficult  to  discriminate  between  an  anomalous  and  a 
pathologic  condition,  the  latter  usually  being  the  case  when  the 
roots  of  individual  teeth  are  affected.  Roots,  to  be  considered 
anomalous  in  size,  should  in  a  measure  retain  their  normal  form, 
cases  of  hypertrophy  (hypercementosis)  usually  resulting  in  the 
destruction  of  the  normal  contours  (Figs.  336  and  2)37)- 


NDEX, 


Accessory  palatal  foramina,  34 
Adipose  tissue,  349 
Alveolar  process,  60-74 

development  of,  64 
Alveoli,  61 

Alveolodental  membrane,  12S-446 
blood  supply  to,  130 
cells  of,  448 
fibers  of,  450 
histology  of,  446 
■    interfibrous  elements  of,  453 
nerve  supply  to,  130 
Amelification,  401 
Ameloblasts,  289,  400,  402,  403 
Anatomic  element,  344 
Angular  artery,  29 
Anomalies  of  the  teeth,  481 
Anterior  palatal  foramen,  34 
nerve,  35 

superior  dental  nerve,  121 
Antrum  of  Highraore,  65 
Apical  foramina,  description  of,  238 
Areolar  tissue,  34S 
Artery,  deep  facial,  34,  49 
inferior  dental,  119 
internal  maxillary,  34,  115 
lingual,  49 
ranine,  50 
sublingual,  50 
Azygos  uvulte,  39 


B. 

Bicuspid,  lower  first,  217 
second,  221 
upper  first,  163 
second,  175 
Blastoderm,  457 
cells  of,  457 
layers  of,  457 
Blood,  course  of,  from  heart  to  cheeks,  30 
from  heart  to  hard  palate,  35 
from  heart  to  lips,  24 
from  heart  to  soft  palate,  39 
from  heart  to  tongue,  50 
Blood  supply  to  the  teeth,  115 
Bone,  351 

canaliculi  of,  352 

cells  of,  352 

Haversian  canals  of,  352 

histologic  examination  of,  353 

byoid,  42,  80 

inferior  maxillary,  71 

lacunae  of,  352 


Bone,  marrow  of,  354 
matrix  of,  351 
osteoblasts  of,  354 
palate,  67 
periosteum  of,  354 
Sharpey's  fibers  of,  354 
superior  maxillary,  51 

Bones,  of  the  mouth,  51 
superior  maxillary,  51 

Brown  striase  of  Retzius,  396 

Buccal  cavity,  457 

embryology  of,  457 
glands,  131,  378 
orifice,  iS 

Buccinator  muscle,  25 


Calcification,  beginning  of,  318 

of  cementum,  333,  439 

of  dentin,  318,  418 

on  enamel,  401 
Calciform  papilte,  43 
Canal,  inferior  dental,  76 

infra-orbital,  52 

posterior  palatal,  55 
Canaliculi,  352 
Canals,  Haversian,  352 

pulp,  23S 
Canine  eminence,  53 

fossa,  53 
Capsular  ligament,  85 
Cartilage,  349 

cells  of,  349 

elastic,  350 

fibro-,  350 

hyaline,  350 

matrix  of,  349 

Meckel's,  79 

permanent,  351 

temporary,  351 

varieties  of,  350 
Cell,  essential  parts  of,  345 

general  description  of,  345 

nucleolus  of,  346 

nucleus  of,  345 

protoplasm  of,  345 
Cells,  ciliated,  347 

columnar,  346 

epithelial,  347 

fat,  349 

gland,  349 

pigment,  349 

plasma,  349 

squamous,  347 


INDEX. 


494 

Cement  corpuscles,  434 

fibers,  436 
Ceraentification,  333,  439 
Cementoblasts,  333,  44^ 
Cementum,  427 

calcification  of,  333,  439 
canaliculi  of,  427 
fibers  of,'  436 
histology  of,  412 
lacunae  of,  427 
lamellae,  431 
matrix  of,  427 
Central  incisor,  lower,  209 

upper,  136 
Cervical  line,  99 
Cheeks,  24 

blood  supply  to,  29 
external  covering  of,  25 
glands  of,  25,  131,  378 
integument  of,  25 
internal  covering  of,  2$ 
mucous  membrane  of,  25,  367 
muscles  of,  25 
muscular  tissue  of,  385 
nerves  of,  30 
substance  of,  25 
Circumvallate  papillfe,  43 
Concrescence  of  teeth,  487 
Condyle,  neck  of,  77 
Condyloid  process,  77-5^3 

forms  of,  83 
Connective  tissue,  cells  of,  346,  347 
classification  of,  347 
fibrous,  347.348 
intercellular  substance  of,  347 
tissues,  347 
Coronoid  process,  77 
Crowns,  anomalous,  485 
Cuspid,  lower,  214 
upper,  153 

D. 

Decalcification,  341 
Deciduous  lower  central  incisor,  279 
cuspid,  280 
first  molar,  275,  281 
lateral  incisor,  273,  279 
second  molar,  278,  283 
teeth,  decalcification  of,  341 
detail  description  of,  271 
enamel  organs  for,  305 
general  description  of,  26S 
occlusion  of,  270 
pulp  chambers  and  canals  of,  2S4 
upper  central  incisor,  271 
upper  cuspid,  274 
Deep  facial  artery,  34,  39  • 

branches  of,  39 
Dendrites,  360 
Dental  arch,  99  . 

arrangement  of  teeth  m,  loi 
curve  described  by,  104 
influence  of  temperament  on,  104 
follicle,  289,  307,  315 
formula,  97 


Dental  furrow,  313 
pulp,  440 
sacculus,  307 
Dentin,  94,  408 

calcification  of,  418 
cells,  305,  307 
chemical  analyses  of,  410 
exposed  by  dissection,  315 
fibers  of,  409,  413 
histology  of,  40S 
matrix  of,  409 
organ,  288,  304 
papillte,  316,  41S 
tubules  of,  409,  410 
Dentinal  fibers,  413 
sheaths,  409,  412 
tubules,  409,  410 
walls  of,  412 
Dentinification,  418 
Dentition,  completion  of,  343 

lack  of,  483 
Depressor  anguli  oris,  28 
labii  inferioris,  23 
labii  superioris,  22 
Development  of  enamel,  397 
of  permanent  teeth,  326 
of  teeth,  286,  470 
Digastric  fossa,  74 
Duct,  parotid,  134 
sublingual,  135 
submaxillary,  135 


Elastic  cartilage,  350 
Elementary  organism,  345 
Embryology,  general,  455 

of  mouth  and  teeth,  455 
Enamel,  94,  3S9 

ameloblasts  of,  400 
brown  stria:  of,  396 
calcification  of,  401 
cells,  289,  400 
chemic  composition  of,  390 
cuticle  of,  445 
development  of,  397 
histology  of,  389 
organ,  288,  289,  398 
cells  of,  289,  297 
external  epithelium  of,  289,  299 
form  of,  2S9,  292 
internal  epithelium  of,  289,  399 
stratum  intermedium  of,  289,  399 
stellate  reticulum  of,  289,  399 
prisms  of;  390 
rods,  formation  of,  404 
Endomysium,  356 
Epithelial  tissues,  346 
Eruption  of  the  teeth,  334 
External  maxillary  artery,  29 
oblique  line,  72 
pterygoid,  92 


Facial  angle,  in 
artery,  29 


495 


Facial  artery,  branches  of,  29 

nerve,  30 

branches  of,  30 
Fat  cells,  349 
Fauces,  anterior  pillars  of,  36 

isthmus  of,  36 

pillars  of,  36 

posterior  pillars  of,  36 
Fibers,  dentinal,  413 

Sharpey's,  354 
Fibroblasts,  449 
Filoro-cartilage,  350 
Fibrous  connective  tissue,  347 

cells  of,  348 
Fifth  nerve,  120 

division  of,  120 
Filiform  papillae,  43 
Floor  of  the  mouth,  boundaries  of,  41 
embryology  of,  460 
framework  of,  40 
Follicle,  dental,  289,  307,  315 
Foramen,  anterior  palatal,  34 

apical,  description  of,  238 

CKCum,  43 

incisive,  59 

infra-orbital,  52,  54 

mental,  73 
Foramina,  accessory  palatal,  34 

posterior  palatal,  34 
Fossa,  glenoid,  8;} 
Frenn;  of  the  mouth,  125 
Fungiform  papillse,  43 
Fusion  of  teeth,  487 

and  concrescence,  4S7 

complete,  488 

partial,  488 


Ganglion,  Gasserian,  120 

Meckel's,  35 

sphenopalatal,  25 
Geminous  teeth,  489 
Genial  tubercles,  73 
Geniohyoglossus,  46 
Germs  for  permanent  molars,  306 
Gingiva;,  33 
Gingival  border,  33 

margins,  outlines  of,  125 
Gland  cells,  349 

follicles,  376 

parotid,  133,  382 

saccular,  376 

tubular,  376 

sublingual,  135,  382 

submaxillary,  134,  3S2 
Glands  and  ducts   of  the  mouth,  histology  of, 
376 

buccal,  131,  378 

excretory  ducts  of,  377 

labial,  131,  377 

Ungual,  132,  379 

molar,  378 

of  the  cheeks,  37S 

of  the  hard  palate,  379 

of  the  mouth,  130,376 


Glands  of  the  soft  palate,  132,  379 

palatal,  132,  379 

salivary,  132,381 
Glenoid  fossa,  S3 
Gomphosis,  96 
Groove,  infra-orbital,  52 
Gubernaculum,  322 

foramina  of,  328 
Gums,  124 

epithelium  of,  369 

fibrous  tissue  of,  369 

general  description  of,  124 

mucous  membrane  of,  125,  369 


H. 

Hard  palate,  31 

arch  of,  ^j 

blood  supply  to,  34 

bones  of,  31 

covering  of,  31 

formation  of,  31 

glands  of,  31,  379 

mucous  membrane  of,  31,  370 

nerves  of,  35 
Haversian  canals,  352 
Highmore,  antrum  of,  65 
Histology,  344 
Hyaline  cartilage,  350 
Hyoglossus,  45 
Hyoid  bone,  40 
Hypoblast,  457 

I. 

Incisive  foramen,  59 

fossa,  53,  72 
Incisor  crest,  59 

lower  central,  209 

lateral,  213 
upper  central,  136 
lateral,  146 
Inferior  coronary  artery,  23,  29 
vein,  30 
dental  artery,  119 

incisive  branch  of,  119 
mental  branch  of,  1 19 
canal,  76 
meatus,  57 
nerve,  120 
lingualis,  4S 
maxillary  bone,  71 
body  of,  72 
development  of,  78 
facial  surface  of,  72 
internal  surface  of,  73 
muscles  attached  to,  78 
vertical  portion  of,  76 
palatal  vein,  35 
turbinated  crest,  54,  58 
Infra-orbital  canal,  52 
foramen,  52 
groove,  52 
Intercellular  substance,  346 
Interglobular  spaces,  147 


496 


Internal  maxillary  artery,  34,  115 

infra-orbital  branch  of  the.  1 17 
superior    maxillary    branch     of 
the,  115 
oblique  line,  73 
pterygoid,  92 
Interproximate  spaces,  106 
Involuntary  muscular  tissue,  355 
Isthmus  of  fauces,  36 


Labial  glands,  131,  377  » 

Lacrimal  canal,  54  • 

tubercle,  55 
Lacunae,  352 
Lateral  incisor,  upper,  146 

lower,  213 
Layer,  granular, 

Malpighi's,  474 
Levator  alseque  nasi,  21 
anguli  oris,  27 
labii  inferioris,  22 
labii  superioris,  21 
palati,  38 
Lines  of  Schreger,  395 
Lingual  artery,  49 
glands,  132,  379 
vein,  50 
Lips,  18 

blood  supply  to,  23 
external  covering  of,  19 
frense  of,  19,  125 
glands  of,  19,  131,  377 
integument  of,  19 
internal  covering  of,  19 
raucous  membrane  of,  19,  y6 
muscles  of,  20 
muscular  tissue  of,  385 
nerves  of,  24  ; 

substance  of,  19 
Lower  bicuspid,  first,  217 

buccal  surface  of,  219 
calcification  of,  217 
cusps  of,  218 
distal  surface  of,  220 
lingual  surface  of,  219 
measurements  of,  217 
mesial  surface  of,  219 
neck  of,  220 
occlusal  surface  of,  21S 
pulp  cavity  of,  264 
root  of,  220 
second,  221 

buccal  surface  of,  222 
calcification  of,  221 
distal  surface  of,  224 
lingual  surface  of,  223 
measurements  of,  221 
.    mesial  surface  of,  223 
neck  of,  224 
'occlusal  surface  of,  221 
root  of,  224 
bicuspids,  general  description  of  the,  217 

pulp  cavities  of,  264 
cuspid,  214 


Lower  cuspid,  calcification  of,  214 

cusp  of,  216 

distal  surface  of,  215 

labial  surface  of,  214 

lingual  surface  of,  215 

measurements  of,  214 

mesial  surface  of,  215 

neck  of,  216 

root  of,  216 
cuspids,  pulp  cavities  of,  263 
incisor,  central,  208 

calcification  of,  208 
cervical  margin  of,  212 
incisor,  cmtting-edge  of,  212 

distal  surface  of,  211 

labial  surface  of,  209 

lingual  surface  of,  210 

mesial  surface  of,  210 

neck  of,  212 

root  of,  212 
incisors,  pulp  cavities  of,  262 
lateral     incisor,    general     description    of, 

213 
molar,  first,  224 

buccal  surface  of,  228 
calcification  of,  224 
cusps  of,  226 
distal  surface  of,  230 
lingual  surface  of,  229 
measurements  of,  224 
mesial  surface  of,  229 
neck  of,  230 
occlusal  surface  of,  225 
roots  of,  230 

second,  231 

buccal  surface  of,  233 
calcification  of,  231 
distal  surface  of,  233 
lingual  surface  of,  233 
measurements  of,  231 
mesial  surface  of,  233 
occlusal  surface  of,  231 
roots  of,  234 

third,  calcification  of,  235 

general  description  of  the,  235 
roots  of,  236 
types  of  the,  237 
molars,  pulp  cavities  of,  265 


M. 

Malpighi's  layer,  474 
Mandible,  71 

evolution  of.  310 
Masseter,  26,  89 
Mastication,  muscles  of,  89 

active  organs  of,  1  7 
Maxillary  bones,  development  of,  309 

sinus,  65 

superior,  51 
Meckel's  cartilage,  78,  310,  466 

ganglion,  35 
Median  raphe,  1 27 
Membrana  eboris,  305 

prreformativa,  403 

propria,  349 


497 


Mental  foramen,  73 

protuberance,  72 
Mesoblast,  457 
Middle  meatus,  57 

superior  dental  nerve,  121 
Molar  glands,  378 
lower,  first,  224 
second,  231 
third,  235 
upper,  first,  177 
second, 191 
third,  200 
Mouth,  17 

angles  of,  18 
bones  of,  51 
boundaries  of,  1 7 
contents  of,  17 
dissection  of,  31 
divisions  of,  31 
entrance  to,  17 
epithelium  of,  364 
floor  of,  3 1 ,  40 
frense  of,  125 
general  description  of,  17 
glands  of,  130 
inferior  portion  of,  40 
interior  of,  31 
lateral  walls  of,  24 
mucous  membrane  of,  126,  364 
muscular  tissues  of,  385 
posterior  boundary  of,  37 
roof  of,  31 
situation  of,  17 
submucosa  of,  365 
superior  portion  of,  31 
tunica  propria  of,  365 
vestibule  of,  17 
walls  of,  24 
Mucous  membrane  of  the  cheeks,  histology  of, 
367 
of  gums,  histology  of,  369 
of  lips,  histology  of,  366 
of  mouth,  126,  364 

blood  supply  to,  365 
histology  of,  364 
nerve  supply  to,  365 
of  tongue,  histology  of,  371 
Muscle,  external  pterygoid,  92 
internal  pterygoid,  92 
masseter,  26,  89 
temporal,  91 
Muscles,  angular  series,  37 
of  mastication,  89 
of  soft  palate,.  37 
of  tongue,  44 
Muscular  tissue,  355 

tissues  of  cheek,  386 

endomysium  of,  356 
involuntary,  355 
of  lips,  38s 

perimysium  of,  356 

sarcolerama  of,  357 

sarcoplasm  of,  357 

of  mouth,  histology  of,  3S5 

non-striated.  355 
of  soft  palate,  386 

32 


Muscular  tissues  of  soft  palate,  striated,  356 
of  tongue,  3S7 

voluntary,  357 
Mylohyoid  ridge,  73 
Myrtriform  fossa,  53 

N. 

Nasal  crest,  59 

spine,  59 
Nasmyth's  membrane,  445  " 

Nerve,  anterior  palatal,  35 
cell,  359 
cells,  363 
corpuscles,  362 
fiber,  axis  cylinder  of,  362 

neurilemma  of,  362 
fibers,  362 

medullated,  362 
non-medullated,  362 
inferior  dental,  122 
inferior  maxillary,  122 
middle  superior  dental,  121 
posterior  superior  dental,  121 
process,  360 
superior  dental,  121 
superior  maxillary,  120 
Nerves,  362 

endoneurium  of,  362 
epineurium  of,  362 
funiculi  of,  362 
medullary  sheath  of,  362 
perineurium  of,  362 
of  tongue,  50 
system  of,  362 
Nervous  tissue,  358 
Neumann's  sheath,  412 
Neurilemma,  362 
Neuroblasts,  359 
Neurocyte,  359 
Neuron,  359 
Non-striated  muscular  tissues,  355 


Occlusion  of  the  teeth,  loS 
of  the  deciduous  teeth,  270 

Odontoblastic  cells,  316 

Odontoblasts,  305,  306,  419,  441 
processes  of,  442 

Oral  cavity,  458 

embryology  of,  45S 

Orbicularis  oris,  20 

Osteoblasts,  354,  447 

Osteoclasts,  449 

Overbite,  III 


Palatal  glands,  132,  379 
raphe,  32,  127 
ruga:,  33,  127 
Palate,  bone,  articulation  of,  70 

attachment  of  muscles  to,  70 
blood  supply  to,  70 
development  of,  70 


INDEX. 


Palate,  bone,  horizontal  plate  of,  67 
vertical  plate  of,  68 
bones,  67 
hard,  31 
soft,  36 
Palatoglossus,  37 
Palatomaxillary  suture,  34 
Palatopharyngeus,  37 
Papilla:  of  the  tongue,  43 
Parotid  duct,  134 

gland,  133,  382 
Perichondrium,  350 
Perimysium,  356 
Periosteum,  354 

Permanent  incisors,  papills  for,  329 
molars,  germs  for,  306 
teeth,  95  f       <; 

preparation  for  development  01,  320 
advance  of,  342 
Pigment  cells,  349 
Pillars  of  fauces,  36 
Plasma  cells,  349 
Posterior  foramina,  34 
palatal  canal,  55 
superior  dental  nerve,  12I 
Primitive  dental  furrow,  313 
Process,  alveolar,  60 
condyloid,  77 
coronoid,  77 
Pulp,  blood-vessels  of,  443 
canals,  238 

cavities,  description  of,  238 
dissections  to  show,  238 
horns  of,  239 
of  deciduous  teeth,  284 
of  lower  teeth,  262 
of  teeth,  238 
of  upper  teeth,  240 
cells  of,  440 
histology  of,  44° 
nerves  of,  444 
odontoblasts  of,  441 


QuADRATUS  menti,  23 


R. 

Ranine  artery,  50 
■  Raphe,  32,  127 
Retzius,  brown  stnse  of,  390 
Ridge,  mylohyoid,  73 
Risorius  muscle,  28 
Roots,  anomalous,  485 

formation  of,  333 
Rugae,  33,  127 

S. 

Sacculus,  dental,  307 
Salivary  glands,  132,  381 

blood-vessels  of,  383 
nerves  of,  383 


Sarcolemma,  351 
Sarcoplasm,  352 
Schreger,  lines  of,  395 
Schwann,  sheath  of,  362 

white  substance  of,  362 
Seventh  nerve,  30 
Sharpey's  fibers,  354 
Sheath,  medullary,  362 

of  Schwann,  362 
Simple  saccular  glands,  376 

tubular  glands,  376 
Soft  palate,  36 

blood  supply  to,  39 
glands  of,  379 
muscles  of,  37 
muscular  tissue  of,  387 
nerves  of,  39 
substance  of,  36 
Spaces,  interglobular,  417 

interproximate,  106 
Sphenomaxillary  ligament,  86 
Sphenopalatal  ganglion,  35 
Stellate  reticulum,  399 
Stenson,  foramen  of,  59 
Stratum  intermedium,  289,  399 
Striated  muscular  tissue,  356 
Styloglossus,  46 
Stylomaxillary  ligament,  85 
Sublingual  artery,  50 
duct,  135 
fossa,  74 
gland,  135 
Submaxillary  duct,  135 
fossa,  74 
gland,  134 
Submucosa,  365 
Supernumerary  teeth,  484 
Superior  coronary  artery,  23,  29 
vein,  30 
lingualis,  48 

maxillary  bone,  articulation  of,  62 
alveolar  process  of,  60 
blood  supply  to,  63 
development  of,  63 
facial  surface  of,  52 
malar  process  of,  58 
muscles  attached  to,  62 
nasal  process  of,  57 
orbital  surface  of,  51 
palatal  process  of,  54,  5^ 
proximal  surface  of,  54 
sinus  of,  55 
tuberosity  of,  56 
zygomatic  surface  of,  56 
bones,  51 
nerve,  120 
meatus,  57 
palatal  vein,  35 
turbinated  crest,  58 
Suture,  palatomaxillary,  34 
Symphysis,  72 


Teeth,  94 

anterior,  99 


499 


Teeth,  apical  extremities  of,  99 

articulation  of,  I09 

attachment  of,  95 

blood  supply  to,  I15,  II9 

classification  of,  95,  96 

complex,  95 

deciduous,  268 

description  in  detail  of,  136 

development  of,  286,  470 

dissections  of,  23S 

division  of,  95 

eruption  of,  334 

names  of,  96 

nerve  supply  to,  120 

occlusion  of,  108 

permanent,  95 

posterior,  99 

pulp  cavities  of,  238 

roots  of,  98 

simple,  95 

surfaces  of,  97 

tissues  of,  389 

veins  from,  119 
Temporal  muscle,  91 
Temporomandibular  aniculation,  82 

movements  of,  88 
Tensor  palati,  38 
Tissue,  adipose,  349 

areolar,  348 

muscular,  355 

nervous,  358 
Tissues  of  the  body,  divisions  of,  346 

connective,  347 

of  the  teeth,  389 
Tongue,  41 

attachment  of,  42 

base  of,  42 

blood-vessels  of,  49 

circumvallate  papilla;  of,  373 

dorsum  of,  42 

embryology  of,  46 

filiform  papillae  of,  371 

frenum  of,  43 

function  of,  41 

fungiform  papilUe  of,  372 

glands  of,  132,  379 

median  raphe  of,  43 

mucous  membrane  of,  371 

muscles  of,  44 

muscular  tissue  of,  3S7 

nerves  of,  50 

papilla:  of,  43 

post-tip,  42 

prebase  of,  43 

shape  of,  42 

size  of,  42 

substance  of,  42 
Tonsil,   36 
Tonsillar  recess,  36 
Tooth  band,  287 

development,  cellular  stage  of,  286, 
470 
saccular  stage  of,  309,  316 

follicle,  walls  of,  308 

fundamental  parts  of  a,  94 
general  description  of  a,  94 


Tooth  germs,  288 

roots,  preparations    for  development  of, 

332 
sac,  307 

sacs  exposed  by  dissection,  314 
sockets,  61 

tissues  of  a,  94 
Transverse  facial  artery,  29 

branches  of,  30 
vein,  30 
Tunica  propria,  365 

U. 

Upper  bicuspid,  first,  163 

angles  of,  169 
buccal  surface  of,  166 
calcification  of,  163 
crown  of,  164 
cusps  of,  165 
distal  surface  of,  169 
measurements  of,  163 
mesial  surface  of,  168 
neck  of,  169 
lingual  surface  of,  167 
pulp  cavity  of,  248 
roots  of,  170 
types  of,  171 

second,  calcification  of,  174 
general  description  of,  175 
measurements  of,  174 
pulp  cavities  of,  251 
cuspid,  153 

calcification  of,  153 

cusp  of,  159 

cutting-edge  of,  15S 

distal  surface  of,  157 

labial  surface  of,  154 

measurement  of,  153 

mesial  surface  of,  157 

neck  of,  159 

Ungual  surface  of,  155 

pulp  cavity  of,  245 

root  of,  160 

types  of,  160 
incisor,  centra],  136 

calcification  of,  136 
crown  of,  137 
cutting-edge  of,  141 
distal  surface  of,  140 
labial  surface  of,  137 
measurements  of,  136 
mesial  surface  of,  139 
neck  of,  142 
lingual  surface  of,  138 
pulp  cavity  of,  240 
root  of,  143 
types  of,  143 

lateral,  146 

calcification  of,  1 46 
crown  of,  147 
cutting-edge  of,  I49 
distal  surface  of,  149 
labial  surface  of,  147 
measurements  of,  146 
mesial  surface  of,  149 


500 


Upper  incisor,  lateral,  neck  of,  156 
libgual  surface  of,  148 
pulp  cavity  of,  243 
root  of,  150 
types  of,  151 
molar,  first,  177 

buccal  surface  of,  184 
calcification  of,  177 
cusps  of,  180 
distal  surface  of,  1S7 
fossEe  and  grooves  of,  I  S3 
lingual  surface  of,  185 
marginal  ridges  of,  179 
measurements  of,  177 
mesial  surface  of,  186 
neck  of,  187 
occlusal  surface  of,  1 78 
pulp  cavity  of,  255 
roots  of,  188 
types  of,  189 
second,  igi 

angles  of,  198 
buccal  surface  of,  196 
calcification  of,  191 
cusps  of,  194 
distal  surface  of,  198 
fossae  and  grooves  of,  195 
lingual  surface  of,  I96 
marginal  ridges  of,  193 
measurement  of,  191 
mesial  surface  of,  197 


Upper  molar,  second,  neck  of,  198 
occlusal  surface  of,  192 
pulp  cavity  of,  258 
roots  of,  198 
third,  200 

buccal  surface  of,  202 
calcification  of,  200 
cusps  of,  205 
distal  surface  of,  202 
fossss  and  grooves  of,  206 
lingual  surface  of,  203 
measurements  of,  200 
mesial  surface  of,  201 
occlusal  surface  of,  204 
pulp  cavity  of,  259 
types  of,  207 

Uvula,  36 


Vein,  inferior  palatal,  35 

lingual,  50 

superior  palatal,  35 
Voluntary  muscular  tissue,  356 


Zygomaticus  major,  27 
minor,  23 


COLUMBIA  UNIVERSITY  LIBRARIES  (hsl.slx) 

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Analiiiiiv.MC  IhMuIduv  ni  Ihrmniilh  and  t 


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1902 

Broomell 
Anatomy  and  histology  of  the  mouth 


